Answer:

Step-by-step explanation:
Let's recall the acceleration definition.
(1)
Δv is the velocity variation, Δv = v(f) - v(i). It is just the difference between final and initial velocities. So Δv = 1.8[m/s] - 5.4[m/s] = -3.6 [m/s]
And Δt is the time variation, in this case it is just 1.5 s.
Therefore, using the equation (1), the acceleration will be:

The minus sign means that the tumbleweed is slowing down.
I hope it helps you!