You need to show your working - all of it. It's impossible to see the errors you've made and the misunderstandings you most likely have.

Here're some things to consider:

f(x + 2) = (x + 2)^2 - 2(x + 2) = expand and simplify.

f(2) = 0.

Therefore f(x + 2) - f(2) = .......

Therefore .