Question

1) How many seconds does it take a car traveling 40 mph, decelerating at a rate of 7 miles per second, to come to a complete stop?

2) How many feet?

Answer 1

a = -7 miles/s^2 I assume you mean but your units look screwy. Perhaps you mean -7 miles per hour/second and I will do that alternative at the end but please read what you wrote in the future before posting.

Vi = 40/3600 miles/second

v = Vi - 7 t

0 = 40/3600 - 7 t

t = .0016 seconds

BUT
if a = -7 miles/hr/s
a = -7 /3600 miles/s^2
and
0 = 40/3600 - 7 t/3600
t = 40/7 = 5.71 seconds

distance = Vi t - (1/2)(-7/3600) t^2

= 40/3600 (5.71) - 3.5 (5.71)^2

3600 d = 228 - 114 = 114 miles

d = 114 miles * 5280 ft/mi /3600
d = 167 feet