Stats help

I can't remember if anyone does / has done the S2 module of A-level maths on here but I'm hoping someone has.

This question is from an old past paper and I'm completely stumped on how to approach it:

A rectangle has perimeter 20cm. The length of one side, X cm, of the rectangle is uniformly distributed between 1cm and 7cm. Find the probability that the length of the longer side of the rectangle is more than 6cm long.

Any help much appreciated. Also the mark scheme for the paper doesn't shed any light on the solution, just the answer.