Physics help?

Two particles of masses m1 and m2 in projectile motion have velocities v1 and v2 respectively at t=0. They collide at t=t1. Their velocities become v1 and v2 at t= 2t1 while still moving in the air. The value of |(m1v1 + m2v2 )-(m1v1+m2v2)| is?
The answer is 2(m1 + m2)gt1. How?
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