Please help with function. BEST ANSWER TO FIRST CORRECT

If f (x) = x^2 − 3x find
(f (x + h)− f (x))/(h)

Please explain because I keep getting it wrong. Thank you so much!

(f (x + h)− f (x))/(h)
=[(x+h)^2-3(x+h)-x^2+3x]/h
simplifying we get
(2xh+h^2-3h)/h
=2x+h-3
if h tends to zero then this represents the differential of the given function f (x) = x^2 − 3x

f(x+h) = (x+h)^2 - 3(x+h)
f(x+h) - f(x) = (x+h)^2 - 3(x+h) - x^2 + 3x
= x^2 + 2xh + h^2 - 3x - 3h - x^2 + 3x
= 2xh + h^2 - 3h
=h(2x + h - 3)
(f(x+h) - f(x))/h = (h(2x + h - 3))/h = 2x + h - 3