Mean Volume of a Pyramid with Random Base

A pyramid with a square base has an altitude of 6 and the side length of its base is chosen at random from the uniformly distributed interval $[2, 6]$. What is the mean volume of the pyramid? The answer is in the form $\frac{\alpha}{\beta}$ where $\alpha$ and $\beta$ are coprime positive integers. Input the value of $\alpha + \beta$.