whatever is inside of the ( ), simply plug that digit into the x-values for f(x)
So: f(x) = 7x - x^2, and f(7+h) - f(7)
= [7(7+h) - (7+h)^2] - [7(7) - (7)^2]
= [49+7h - 49+14h+h^2] - [49-49]
= 49-49 + 7h+14h + h^2 = h^2 + 21h =
h (h+21), h (h+21) = 0
h=0... But it stated h cannot = 0
So h+21 = 0, h = -21
