Question

a car slows down at -5.00m/s squared comes to a stop after 15 m what was the initial speed of the car.

Answer 1

The distance traveled by a car to the rest

$$d=\frac{v_i^2}{2a}$$

Hence, the initial speed of a car

$$v_i=\sqrt{2ad}=\sqrt{2*5.00*15}=12.2\;\rm m/s$$