Two carts, A and B, are connected by a rope $39\,ft$ long that passes over a pulley $P$. The point $Q$ is on the floor $12\,ft$ directly beneath $P$ and between the carts. Cart A is being pulled away from $Q$ at a speed of $2\,ft/s$.
How fast is cart B moving toward $Q$ at the instant when cart A is $5\,ft$ from $Q$?

$ft/s$