PLEASE HELP, OVERDUE ASSIGNMENT!!

Cassy shoots a large marble (Marble A, mass: 0.06 kg) at a smaller marble (Marble B, mass: 0.03 kg) that is sitting still. Marble A was initially moving at a velocity of 0.7 m/s, but after the collision it has a velocity of −0.2 m/s. What is the resulting velocity of marble B after the collision? Be sure to show your work for solving this problem along with the final answer.
I need help because this doesn't make sense to me!! -O-
Please explain in not-gibberishhh! TvT
(It's not multiple choice)