f(x) = x^2 + 4x - 5
[f(a + h) - f(a)] / h
f(a) = a^2 + 4a - 5
f(a + h) = (a + h)^2 +4(a + h) - 5
= a^2 + 2ah + h^2 + 4a + 4h - 5
f(a + h) - f(a) = a^2 + 2ah + h^2 + 4a + 4h - 5 - ( a^2 + 4a - 5)
---------------- ------------------------------------------------------------
h h
= 2ah + h^2 + 4h h (2a + h + 4) 2a + h + 4
------------------- = ------------------- =
h h
