Conservation of energy physics problem

Question

Two boxes are connected over a pulley and held at rest as shown below. Box A has a mass of 15 kg and box B has a mass of 12 kg. If the bottom of box A is originally 85 cm above the floor, with what speed will it contact the floor when the system is released? Use conservation of energy and assume that friction is negligible. 

Answer 1

Since there is no diagram, I assume that there are jus two masses hanging on either side of a pulley - Atwood's Machine). Let h be the height that the 12 kg mass goes up (same distance 15 kg goes down:

Conservation of energy of the entire system is that the negative change in potential energy = change in kinetic energy

The negative change in potential is (m_A-m_B)gh

The change in kinetic energy = 1/2 (m_A + m_B) v²

Equate the 2 terms and solve for v in terms of h and the two masses.