Physics question about speed & collision

Dancer, who has a mass of 7 kg, ran at 3 m/s directly at Prancer who has a mass of 10 kg. If both reindeer come to a stop after the collision, what was Prancer’s velocity before the collision? In general, how do I solve this type of questions? Please explain and give me at least 5 more related sample quesitons with explanations.