AHSME 1970 Problem 28

In triangle $ABC$, the median from vertex $A$ is perpendicular to the median from vertex $B$. If the lengths of sides $AC$ and $BC$ are $6$ and $7$ respectively, then the length of side $AB$ is $\text{(A) } \sqrt{17} \text{(B) } 4 \text{(C) } 4\tfrac{1}{2} \text{(D) } 2\sqrt{5} \text{(E) } 4\tfrac{1}{4}$