b(X) = X^2 + 3

find / simplify b(a+h) -b(a) / h

I thought I knew how to do this, but the function of X is confusing me since there is no X variable in the Find.

$\frac{(a+h)^2 + 3 - (a^2+3)}{h}$
expand the numerator and combine like terms, then factor ... you should get the $h$ in the denominator to divide out