In Exercises $1-8,$ compute each expression, given that the functions $f, g, h, k,$ and $m$ are defined as follows:
$$\begin{array}{ll} f(x)=2 x-1 & k(x)=2, \quad \text { for all } x \\ g(x)=x^{2}-3 x-6 & m(x)=x^{2}-9 \\ h(x)=x^{3} \end{array}$$
(a) $(f g)(x)$
(b) $(f g)(1 / 2)$