Question 1028661
If f(f(x)) = f(x) then we need to find every the solution to f(x) = x

X^2 -2x = x is equivalent to x^2-2x-x = 0
                             X^2 -3x = 0
                             X(x-3) =0
 So, solutions are x=0 or x=3.

1) f(f(0)) =f(0^2-2×0) =f(0) (checked)
2) f(f(3)) = f(3^3-2×3) = f(9-6) = f(3) (checked)