Find (f*g)(x)

Plug g(x) into the x in f(x)

$(f\cdot g)(x) = f(x) \cdot g(x) = 6x(x + 5) = 6x^2 + 30x$

But if you meant $(f\circ g)(x)$:

$(f\circ g)(x) = f(g(x)) = 6 g(x) = 6(x + 5) = 6x + 30$