Step-by-step explanation:
constant acceleration creates an arithmetic sequence of velocities with the quantity of that acceleration being the constant term d.
a0 = 13.5 m/s
d = 1.9 (m/s²)
we know
an = a0 + n×d
that is actually the reason for the first equation of motion :
v = u +at.
v is the final velocity (after a given time t), u is the initial velocity, a is the acceleration.
it is one and the same as our arithmetic sequence rule.
so, the velocity of the car after the first 6.2 seconds is
v = 13.5 + 6.2×1.9 = 25.28 m/s
now, it goes the other way around.
with that velocity as initial velocity, how long is the time to reach 0 ?
this time we know v (0), and we need to calculate t :
0 = 25.28 - 1.2×t
-25.28 = -1.2×t
t = -25.28/-1.2 = 21.06666666... seconds
it seems everything here is rounded to tenths, so it takes about 21.1 seconds to stop.
the total time for the whole action is then this braking time plus the original 6.2 seconds of acceleration :
total time = 6.2 + 21.1 = 27.3 seconds