To prove it's a contraction you proceed as follows:

now taking the supremum over we get that this last is

To find the fixed point you could follow th proof of the fixed point theorem and iterate given a simple initial function (say or some such) or you could try by inspection see if you can find it.

Edit: For simplicity you could assume is differentiable, derive the expression and obtain an easy diff. equation, solve, substitute and determine the constant.

Spoiler: