A hockey player has an acceleration of - 3.1m/s2 how long does it take him come to a complete stop from a speed of 13m/s

Since his acceleration is not changing AND the acceleration is negative, we know that the hockey player is slowing down at a regular rate (i. e losing 3.1m/s, every second).

It's simple to then see that you just need to work out how many times 3.1 goes in to the starting velocity of 13m/s.

13 / 3.1 = 4.19 seconds