A 0.4-kg cart moving in the positive x-direction crashes head-on into a 0.5-kg cart that is initially at rest. What is the velocity of the 0.4-kg cart after the elastic collision?
(I know that to solve this problem, I have to solve for the final velocity of the 0.5-kg cart in the conservation of momentum equation, then substitute it into the conservation of kinetic energy equation and solve for the final velocity of the 0.4-kg cart. However, I'm having trouble simplifying the conservation of kinetic energy equation once I make the substitution. Any tips?)

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