Answer:
18x +9h
Step-by-step explanation:
Such problems are tedious, but not difficult. The trick is to avoid making mistakes in the algebra.
[tex]\dfrac{f(x+h)-f(x)}{h}=\dfrac{(9(x+h)^2)-(9x^2)}{h}\\\\=\dfrac{9(x^2+2xh+h^2)-9x^2}{h}=\dfrac{18xh+9h^2}{h}=\boxed{18x+9h}[/tex]
