Please help me do this physics problem. I don’t have a clue what formula to use.?

Question

An 8.9kg mass (M1) is attached to a light cord that passes over a massless, frictionless pulley. The other end of the cord is attached to a 3.9kg mass (M2). The acceleration of gravity is 9.8 m/s^2.
See pic http://www.fearofphysics.com/Probs/Thumbs/mech040fig01.jpg

1) Use the conservation of energy to determine the final speed of the first mass after it has fallen (starting from rest) 7.2m. (Answer in units of m/s

Answer 1

You have a mass difference of 5 kg and a sum of 12.8 kg.
The acceleration a = 5*g/12.8.
s=0.5*a*t^2 = v^2/(2*a), so
v=sqrt(2*a*s) = sqrt(2*5*9.81/12.8 * 7.2) = 7.43 m/s.

Whoops, we're supposed to use conservation of energy. Well, answer 1's kinetic energy equation needs to be adjusted for both masses having the same final velocity, and there is an algebraic error in the last equation (m1+m2 should be m1-m2). Thus
Ef = 0.5*(m1+m2)*v^2, and
v=sqrt(2*g*h(m1-m2)/(m1+m2)) = 7.43 m/s.

Answer 2

Ei=Ef
For M1: Ei=(M1*g*h)-(M2*g*h); Ef=.5*M1*v^2
V=sqrt(2gh(M1+M2)/M1)