Find a if the coefficients of $x^2$ and $x^3$ in the expansion of $(3+ax)^9$ are equal.

Question

$\begin{array}{1 1}(A)\;\large\frac{9}{4}\\(B)\;\large\frac{9}{5}\\(C)\;\large\frac{9}{7}\\(D)\;\large\frac{9}{8}\end{array} $