Using related rates to answer a pulley question.

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

Express solution using related rate notation i.e. dy/dt or dx/dt