Two carts $($each with same mass $10$ kg$)$ collide on a track and after collision, they stick together.$$ If the initial velocity of the $1$st cart $=10$m$/$s right and its final velocity is $5$m$/$s left,$$

Prompt $1$what was the $2$nd cart$$s final velocity?

Answer for prompt $1$ what was the $2$nd cart$$s final velocity?

Prompt $2$what was the $2$nd cart$$s initial velocity?