You are driving at a speed of 20 m/s. suddenly you see a deer 100m in front of the car and hit the brakes immediately. You are just able to save the deer. (a) calculate the acceleration. (b) how many seconds did you need to stop the car?

Use the formula:

v^2 = u^2 + 2as

v is the final speed of the car
u is the initial speed of the car
a is the acceleration
s is the displacement.

For the second part, use

v = u+at

The symbols are the same, and t represent time.