First, let us set-up the system, and identify the variables:
The initial velocity, `v_i` , is `45 m/s` .
The final velocity, `v_f` , is `0 m/s` .
The acceleration,`a` , is `-20 m/s^2` . Note the negative sign to denote decceleration.
We want to find `t` , the time it takes, given the acceleration, to reach the final velocity from the initial velocity -- essentially, how long before the car stops.
The formula that we need is as follows:
`v_f = v_i + at`
Isolating `t` ,
`t = (v_f - v_i)/a`
Then, we simply plug-in the values:
`t = (0 - 45)/(-20) = 2.25`
That is, it takes 2.25 seconds before the car stops.