A car brakes from 19 m/s to a stop in 40 meters. What is the acceleration of the car?

Speed and Distance given Acceleration

v = vo + a t
d = vo t + 0.5 a t2

t = time
v = speed at time t
vo = speed when t = 0 a = acceleration (-a for deceleration)
d = distance traveled at time t

Many safety experts use 15 ft/sec2 (0.47 g's) as the maximum deceleration that is safe for the average driver to maintain control, good to excellent tires, dry surface. A reasonably skilled driver can stop at 20 ft/sec2 (0.62 g's). Most production street vehicles have a maximum braking around 0.8 g's.

Maximum Braking for select Vehicles
Productions years 1991 - 1995. Braking to a stop
measured on a dry flat surface starting at 60 mph.
Vehicleg'sft/s2mph/s
BMW M31.032.322.0
Toyota Celica GT0.9430.220.6
Lincoln Continental0.9229.620.2
Nissan Maxima0.8527.318.6
Chevy Blazer0.7624.516.7
Dodge Colt GL0.7223.215.8

Full change in the electric plus system