what acceleration is needed to stop in time from hitting the wall?

chris drives a 1200 kg car straight at a brick wall. he is accelerating at a rate of 4 m/s^2. he started from rest and accelerated for 3 seconds, and began to hit the brakes when he was 9m away from the wall. 
 
what acceleration is needed to stop in time from hitting the wall?

he must bring his speed down to zero over a distance x = 9m.
 
v22 - v12 = 2ax
 
v2 = 0 ⇒
 
a = -v12/(2x)
 
v1 = a0t = (4 m/s2)(3s) = 12 m/s
 
a = -(12 m/s)2 / [2(9m)] = -8 m/s2
 
the minus sign means the acceleration is applied opposite to the direction of motion.