Let $\>l\>$ be the length of a diagonal of a rectangle whose sides have lengths $\>x\>$ and $\>y\>$, and assume that $\>x\>$ and $\>y\>$ vary with time.
If $\>x\>$ increases at a constant rate of $\> \frac {1} {9}\>$ ft/s and $\>y\>$ decreases at a constant rate of $\> \frac {1} {7}\>$ ft/s, how fast is the size of the diagonal changing when $\>x = 6\>$ ft. and $\>y = 7\>$ ft?