The best way to do the difference quotient is to do it in steps.
If it contains no square roots of variables:
1. Find f(a+h) and simplify if possible.
2. Find f(a+h)-f(a) and simplify if possible. Every term that
contains no h should cancel. Factor out h if necessary.
3. Find [f(a+h)-f(a)]/h and cancel the h factor. If step 2 is
a fraction, change division by h to multiplication by .
[If it contains a square root, multiply and divide by the conjugate first. You don't have that here]
1.
2.
Get LCD of (a+h+4)(a+4)
3.
Edwin