Answer
$v = 16.0 ~m/s$
Work Step by Step
The change in velocity will be equal to the area between the acceleration versus time graph and the x-axis;
$\Delta v = \frac{1}{2}(4.0~m/s^2)(4.0~s)$
$\Delta v = 8.0~m/s$
We can find the velocity at t = 4.0 s;
$v = v_0 + \Delta v$
$v = 8.0~m/s + 8.0~m/s$
$v = 16.0 ~m/s$
