Answer
a) $(fg)(x)=36x^3-9x^2$
b) $(fg)(-1)=-45$
Work Step by Step
a) Using $(fg)(x)=f(x)g(x),$ then with $f(x)=12x^2-3x$ and $g(x)=3x$,
\begin{array}{l}\require{cancel}
(fg)(x)=(12x^2-3x)(3x)
.\end{array}
Using $(b+c)a=ab+ac$ or the Distributive Property, the expression above is equivalent to
\begin{array}{l}\require{cancel}
(fg)(x)=12x^2(3x)-3x(3x)
\\\\
(fg)(x)=36x^3-9x^2
.\end{array}
b) Replacing $x=-1$ in $(fg)(x)=36x^3-9x^2,$ then $(fg)(-1)$ evaluates to
\begin{array}{l}\require{cancel}
(fg)(-1)=36(-1)^3-9(-1)^2
\\\\
(fg)(-1)=36(-1)-9(1)
\\\\
(fg)(-1)=-36-9
\\\\
(fg)(-1)=-45
.\end{array}
