A boy drops a stone vertically down from the margin of a

Question

Problem: A boy drops a stone vertically down from the margin of a 24-m cliff. At the same moment, his friend at the bottom of the cliff throws another stone vertically upward with an initial velocity of 12 m/s.

Part A: After how many seconds will the two stones pass each other, given gravitational acceleration g=10 m/s².

I would really appreciate some help for above problem.