An object of mass 5kg moving with a velocity of 6m/s collides with another object of mass 3kg moving with a velocity of 4 m/s. After collision both bodies stuck together and move with a common velocity. Find the velocity?

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