This problem is concerned with the composition of functions, and the fact that this is different from the multiplication of functions. If $f$ and $g$ are two functions then their product $fg$ is defined by $$(fg) (x) = f(x) g(x).$$ As usual, the absence of an arithmetic operator denotes multiplication.
The composition $f\circ g$ of these two functions is quite different. We evaluate first one, and then the other. Thus $$(f\circ g)(x) = f\big(g(x)\big).$$

Let $f(x) = 3 x + 5$ and $g(x) = 3 x^2 + 7 x$.

$(fg)(8) =$