Two carts move towards each other at a speed of 4 m / s each. After the collision, the second bogie got a speed in the direction

Two carts move towards each other at a speed of 4 m / s each. After the collision, the second bogie got a speed in the direction of movement of the first bogie equal to 6 m / s, and the first stopped. Calculate the mass of the first cart if the mass of the second is 2 kg.

We need a law of conservation of momentum
m1v1 + m2v2 = m1v1 ‘+ m2v2’
But we have different directions of the carts, we will assume that the first one goes in a positive direction
In the second case, the speed of the first carriage = 0, so that m1v1 ‘= 0, then
m1v1-m2v2 = 0 + m2v2 ‘
You can immediately substitute, or you can express. I will express
m1 = (m2v2’-m2v2) / v1 = (2 * 6 + 2 * 4) / 4 = 5 kg
Answer: 5 kg