AHSME 1961 Problem 36

In $\triangle ABC$ the median from $A$ is given perpendicular to the median from $B$. If $BC=7$ and $AC=6$, find the length of $AB$. $\textbf{(A)}\ 4 \textbf{(B)}\ \sqrt{17} \textbf{(C)}\ 4.25 \textbf{(D)}\ 2\sqrt{5} \textbf{(E)}\ 4.5$