for mass m

mgl=1/2m v1^2

 find initial velocity v1

similarly

2mgl=1/2mv2^2

 

v1 and v2 will be the velocities of m and 2m

use momentum conservation and the

e= relative velocities after collision/rel velocities before collision

u will get two equations and two variable

solve it to get the vel  after collision

for the seccond part use energy conservation

to find the  heights