For the function 2x-x^2

find f(4)-f(2) / (2)

I got -8-0/2 so my final answer Is -4

assume you mean:

[ f(4)-f(2) ] / (2)

f(x) = x(2-x)

if x = 4
4(2-4) = -8

if x = 2
2(2-2) = 0
so we have
-8/2 = -4