math problem

i am trying to find f'(a) for f(x) = 2 - 7x + 8x^2.

I used the formula (f(a + h) - f(a))/h and got to (7h+16ah+8h^2)/h the problem is that the h^2 doesn't cancel out, I really don't know how to get rid of it.

f'(x)=-7+16x
f'(a)=-7+16a
bam

Well, cancelling the h's gives you -7 + 16a + 8h.  As you're taking the limit as h approaches 0, you can now substitute in 0, leaving you with f'(a) = -7 + 16a, as said by the previous poster.

Silly kids.

Collect. I thought you learn this in middle school ?