Using the limit definition, find f'(-1) if f(x) = 1 - x2.
We have a = -1 and f(x) = 1 - x2, so we find
This next step is a dangerous one. When evaluating f(-1 + h), make sure to plug in all of (-1 + h) for x, and don't forget the rest of the function f.
We conclude f'(-1) = 2.