A cart of mass M = 1.70 kg can roll without friction on a level track. A light string draped over a light, frictionless pulley connects the cart to a hanger with mass m = 1.25 kg. The cart is released from rest, allowing it to roll, pulled by the falling hanger. What is the formula for the acceleration a of the cart in terms of the cart's mass M, the hanger's mass m, and the acceleration of gravity g?