Hi there!
Recall the conservation of momentum:
m1v1 + m2v2 = m1v1' + m2v2'
Let m1 = 5 kg ball and m2 = 10 kg ball
Since m2 is at rest, we can rewrite:
m1v1 = m1v1' + m2v2'
Plug in the given values:
5(2) = 5(-1) + 10v2'
Solve for v2':
10 = -5 + 10v2'
15 = 10v2'
15/10 = 1.5 m/s