ANSWER
The turning point is

EXPLANATION
The function given to us is

At turning point,

So we need to differentiate the given function and equate it to zero.
We using the chain rule of differentiation, we obtain,

We equate this to zero to obtain,

We divide through by 3.

We solve for x to get,


We substitute this x-value in to the function to obtain the corresponding y-value of the turning point.



Therefore the turning point is

C is the correct answer.
the answer is 8! hope that helps
Is the question asking f(g(x))? If so the answer is 4x+20