Combine the given two facts to write a concluding fact. Note that there should be some parts of the first and second facts that are not mentioned in this conclusion fact. Your combined fact should be the result of a chain between the two facts. Chains form when two facts connect together to produce a concluding fact. An example of a chain is: "pesticides cause pollution" (fact1) + "pollution can harm animals" (fact2) → "pesticides can harm animals" (conclusion fact). Parts of the concluding fact overlap with the first and the second fact. To construct such a  concluding fact, a useful way is to borrow the subject from one fact and the ending conclusions from another fact.
--------
Question: Fact 1: using a gasoline engine requires burning gasoline. 
Fact 2: Gasoline and flammable liquid related burns, however, are a preventable problem.

Answer: using an engine requires burning flammable liquid.


Question: Fact 1: using a gasoline engine requires burning gasoline. 
Fact 2: Oil and gasoline are fossil fuels.

Answer: Many engines burn fossil fuels.


Question: Fact 1: a valley is formed by a river flowing. 
Fact 2: Fast flowing rivers tend to pick up more pebbles, sand and other materials that cause erosion.

Answer:
A valley are formed by erosion.