We use kinematic equations,
During reaction time,
(A)
During the deceleration,
. (B)
Here, u is initial velocity, v is final velocity , a is acceleration and t is time.
There is no deceleration during his 0.8 sec reaction time so a=0, from equation (A),
During the deceleration (a = -7.0 m/s^2) , final velocity will be zero i.e v = 0, therefore from equation (B),
Thus, the distance traveled by car after he sees need to put brakes is