marathon oil corporation notes to consolidated financial statements operating lease rental expense was : ( in millions ) 2008 2007 2006 minimum rental ( a ) $ 245 $ 209 $ 172 .
[['( in millions )', '2008', '2007', '2006'], ['minimum rental ( a )', '$ 245', '$ 209', '$ 172'], ['contingent rental', '22', '33', '28'], ['sublease rentals', '2013', '2013', '-7 ( 7 )'], ['net rental expense', '$ 267', '$ 242', '$ 193']]
( a ) excludes $ 5 million , $ 8 million and $ 9 million paid by united states steel in 2008 , 2007 and 2006 on assumed leases .27 .contingencies and commitments we are the subject of , or party to , a number of pending or threatened legal actions , contingencies and commitments involving a variety of matters , including laws and regulations relating to the environment .certain of these matters are discussed below .the ultimate resolution of these contingencies could , individually or in the aggregate , be material to our consolidated financial statements .however , management believes that we will remain a viable and competitive enterprise even though it is possible that these contingencies could be resolved unfavorably .environmental matters 2013 we are subject to federal , state , local and foreign laws and regulations relating to the environment .these laws generally provide for control of pollutants released into the environment and require responsible parties to undertake remediation of hazardous waste disposal sites .penalties may be imposed for noncompliance .at december 31 , 2008 and 2007 , accrued liabilities for remediation totaled $ 111 million and $ 108 million .it is not presently possible to estimate the ultimate amount of all remediation costs that might be incurred or the penalties that may be imposed .receivables for recoverable costs from certain states , under programs to assist companies in clean-up efforts related to underground storage tanks at retail marketing outlets , were $ 60 and $ 66 million at december 31 , 2008 and 2007 .we are a defendant , along with other refining companies , in 20 cases arising in three states alleging damages for methyl tertiary-butyl ether ( 201cmtbe 201d ) contamination .we have also received seven toxic substances control act notice letters involving potential claims in two states .such notice letters are often followed by litigation .like the cases that were settled in 2008 , the remaining mtbe cases are consolidated in a multidistrict litigation in the southern district of new york for pretrial proceedings .nineteen of the remaining cases allege damages to water supply wells , similar to the damages claimed in the settled cases .in the other remaining case , the state of new jersey is seeking natural resources damages allegedly resulting from contamination of groundwater by mtbe .this is the only mtbe contamination case in which we are a defendant and natural resources damages are sought .we are vigorously defending these cases .we , along with a number of other defendants , have engaged in settlement discussions related to the majority of the cases in which we are a defendant .we do not expect our share of liability , if any , for the remaining cases to significantly impact our consolidated results of operations , financial position or cash flows .a lawsuit filed in the united states district court for the southern district of west virginia alleges that our catlettsburg , kentucky , refinery distributed contaminated gasoline to wholesalers and retailers for a period prior to august , 2003 , causing permanent damage to storage tanks , dispensers and related equipment , resulting in lost profits , business disruption and personal and real property damages .following the incident , we conducted remediation operations at affected facilities , and we deny that any permanent damages resulted from the incident .class action certification was granted in august 2007 .we have entered into a tentative settlement agreement in this case .notice of the proposed settlement has been sent to the class members .approval by the court after a fairness hearing is required before the settlement can be finalized .the fairness hearing is scheduled in the first quarter of 2009 .the proposed settlement will not significantly impact our consolidated results of operations , financial position or cash flows .guarantees 2013 we have provided certain guarantees , direct and indirect , of the indebtedness of other companies .under the terms of most of these guarantee arrangements , we would be required to perform should the guaranteed party fail to fulfill its obligations under the specified arrangements .in addition to these financial guarantees , we also have various performance guarantees related to specific agreements. .

Question: by how much did net rental expense increase from 2006 to 2008?
Answer: 38.3%