Answer:
a = -1.833 m/s²
Step-by-step explanation:
Acceleration is the change in velocity over change in time, so you will use a = Δv/Δt or you can also use a = (v-v0)/t, both means the same thing, and I will show you how to use the latter equation.
a = average acceleration
v = final velocity = 7 m/s
v0 = initial velocity = 18 m/s
t = time = 6 s
a = (v-v0)/t
a = (7m/s - 18m/s) / 6s
a = -1.833 m/s²