Take the moment Allen sees the moose to be the origin.
First, convert his speed to m/s.
90 km/h = (90 km/h) • (1000 m/km) • (1/3600 h/s) = 25 m/s
(a) For the time it takes him to react (0.85 s), Allen is moving at a constant speed of 25 m/s, so that before he actually does anything, he covers a distance of
(25 m/s) • (0.85 s) = 21.25 m
(b) Once he presses the brakes, Allen's vehicle covers a distance x in time t of
x = 21.25 m + (25 m/s) t + 1/2 a t²
and has a speed v of
v = 25 m/s + a t
It takes him 3.5 s to come to a full stop. Use this to find the acceleration:
0 = 25 m/s + a (3.5 s)
a = - (25 m/s) / (3.5 s)
a ≈ - 7.1 m/s²
After 3.5 s, he will have traveled a total distance of
x = 21.25 m + (25 m/s) (3.5 s) + 1/2 (- 7.1 m/s²) (3.5 s)²
x = 152.5 m ≈ 150 m
(c) This one is worded a bit strangely, specifically "once he presses the brake" seems to suggest instantaneous acceleration, not average. Average acceleration is defined for some duration of time. You're probably expected to report the acceleration of the car as it comes to a stop, which we found earlier to be
a ≈ - 7.1 m/s²