Fixed point : error of estimation

I'm just stuck on this problem :

We want to solve via a function of iteration that satisfies the necessaries conditions of convergence. Determine a bound of the error which is committed in each step in function of the difference between the 2 last values of the sequence. In other words, determine such that . Where is the zero that we are looking for.

Tip : Use the triangular inequality in the inverse sens than usual.

I don't know how to start. I'm stuck at starting it and I need it to solve the second part of the exercise.