Complex sentence: Rebecca became an active environmentalist at the age of 20 while she was studing politics at Bristol University when she joined the protest at Twyford Down in 1992 against the M3 motorway which was being built close to where she grew up.
Split into simpler sentences: Rebecca became an active environmentalist at the age of 20 while she was studying politics at Bristol University. In 1992 she joined the protest camp at Twyford Down against the construction of a section of the M3 motorway being built close to where she grew up.