Mark M. answered 09/03/19
Retired math prof. Calc 1, 2 and AP Calculus tutoring experience.
f(x) = -7x2 + 10x
f(x+h) = -7(x+h)2 + 10(x+h) = -7(x2 + 2xh + h2) + 10x + 10h
f(x+h) - f(x) = -14xh - 7h2 + 10h
[f(x+h) - f(x)] / h = -14x - 7h + 10
f'(x) = limh→0[(f(x+h) - f(x)) / h] = limh→0(-14x - 7h + 10) = -14x + 10