The problem is that the difference quotient is a function in a (or h). So think of your first equation as a template. If you want to figure out the slope of the tangent line to (2, 0), plug in 2 for x in the difference quotient to obtain:
f\prime(2) = \lim_{a\rightarrow 2}\frac{f(2) - f(a)}{2 -...