If f(x)=3x^2+8 and g(x)=4x+21, then what is (f times g)(-5)? The answer is 11, but I do not know how to work it out.

That is not a 'times'. It is a composition. Find f(g(x))

or maybe it's written as f\circ g

That little circle is not a times symbol. It means composition.

That means sub one function in for x into the other.

Like so:

3(4x+21)^{2}+8

Now, plug in x=-5

By saying 'times' it means (3x^{2}+8)(4x+21) which is entirely different and not what the problem means.