\textbf{134-} An isosceles trapezoid with leg length $a$ and $6$ bases, on a circle with area $15\pi$ is circumscribed. The perimeter $a$ is equal to which value?
\[
\text{(1)}\ \frac{25}{3} \qquad \text{(2)}\ 8 \qquad \text{(3)}\ \frac{33}{3} \qquad \text{(4)}\ 15
\]