Answer
$a = -2.0~m/s^2$
Work Step by Step
The acceleration is the slope of the velocity versus time graph.
We can consider the two points on the graph $(4.0~s, 4.0~m/s)$ and $(6.0~s, 0)$
We can find the slope at $t = 5.0~s$:
$a = \frac{\Delta v}{\Delta t} = \frac{0-4.0~m/s}{2.0~s} = -2.0~m/s^2$
