Two 125 kg bumper cars are moving toward each other in opposite directions. car x is moving at 10 m/s and car z at –12 m/s when they collide head–on. if the resulting velocity of car z after the collision is 10 m/s, what is the velocity of car x after the collision?
–12 m/s
–10 m/s
10 m/s
12 m/s

Could someone . i have tried to solve myself but my calculation is off

You are given two vectors a⃗ =−3.00ι^ 5.00j^ and b⃗ =5.00ι^ 2.00j^. let the counterclockwise angles be positive.

Assume you measured the mass of the cart to be (500 ± 1) g and the mass of the additional mass you put on the cart to be (500 ± 1) g as well. since the scale you are using in the lab cannot measure objects heavier than 600g you will have to sum up individual pieces and propagate the error. so what would be the mass and the standard error of the cart and the mass