f(x) = 4x2
[f(x+h) - f(x)]/h = [4(x+h)2 - 4x2]/h
= [4(x2 + 2xh + h2) - 4x2]/h
= [4x2 + 8xh + 4h2 - 4x2]/h
= [8xh + 4h2]/h
= [h(8x + 4h)]/h
= 8x + 4h