Question

a car is traveling 3m/s decelerates at 7m/s*2. How long is it before it comes to the rest?

Answer 1

The velocity of the car which moves with the constant (and negative, because the car is decelerating) acceleration is:

$$v = v_0 - at$$

where $v_0 = 3m/s$ is the initial speed, $t$ is the time and $a = 7m/s^2$ is the acceleration.

At rest $v = 0$. Thus:

$$0 = v_0 - at\\ t = \dfrac{v_0}{a} = \dfrac{3}{7}s\approx 0.43s$$

Answer. 3/7s or 0.43s.