For this we can use equations for motion.

V = at + V0

V  = final velocity

a  = accelaration

t  = time

V0= Initial velocity

For our question;

V  = 0 mi/s

a  = ????

t  = 9 s

V0= 60 mi/hr = 60/(3600) mi/s = 1/60 m/s

V = at + V0

a = (V-V0)/t

  = (0-1/60)/9 m/s^2

  = 0.00185 mi/s^2

The constant negative accelaration (deccelaration) of the car is 0.00185 mi/s^2.

The initial speed of the car is 60 mi/hr or `60/3600` mi/s. It has to come to a halt in 9 seconds.

The required negative acceleration for this to happen is `(0 - 1/60)/9 = 1/540` mi/s^2

The required acceleration to bring the car to a halt in 9 s is -1/540 mi/s^2.