The pyramid!

Shown above is a regular frustum of a pyramid with regular hexagonal bases. If the edge of the lower base is $9$ , the edge of the upper base is $4$ , and the length of each lateral edge is $12$ , find its volume. If your answer can be expressed as $\dfrac{a}{2}\sqrt{b}$ , where $a$ and $b$ are positive co-prime integers, submit to the answer box the value of $a+b$ .