AHSME 1961 Problem 3

If the graphs of $2y+x+3=0$ and $3y+ax+2=0$ are to meet at right angles, the value of $a$ is: $\textbf{(A)}\ \pm \frac{2}{3} \textbf{(B)}\ -\frac{2}{3} \textbf{(C)}\ -\frac{3}{2} \textbf{(D)}\ 6 \textbf{(E)}\ -6$