Can someone help with a physics question please

Two carts with masses of 10.3 kg and 2.8 kg
move in opposite directions on a frictionless
horizontal track with speeds of 7.4 m/s and
2.7 m/s, respectively. The carts stick together
after colliding head-on.
Find their final speed.
Answer in units of m/s
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A boy on a 1.9 kg skateboard initially at rest
tosses a(n) 8.0 kg jug of water in the forward
direction.
If the jug has a speed of 2.9 m/s relative to
the ground and the boy and skateboard move
in the opposite direction at 0.57 m/s, find the
boy’s mass.
Answer in units of kg

Principle of Conservation of Linear Momentum:

M1 + M2 = M3
M1V1 + M2V2 = (M1+M2)V3
V3 = (M1V1+M2V2)/(M1+M2)

Assume positive direction of velocity is the original direction of movement of the 10.3kg cart and that no energy is dissipated in the collision of the two carts:

V3 = (10.3*7.4 - 2.8*2.7)/(10.3+2.8)
V3 = 5.24 m/s (in the original direction of the 10.3kg cart)

Final velocity of the joined carts is 5.24 m/s in the original direction of the 10.3kg card
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0 = MjVj + (Mb+Ms)Vbs
Mb + Ms = -MjVj/Vbs
Mb = -MjVj/Vbs - Ms
Mb = -(8*2.9)/(-.57) - 1.9
Mb = 38.8 kg

Mass of the boy is 38.8 kg