Answer
The answer is
$$\vec{v}_m=(4\text{ m/s })\hat{x}.$$
Work Step by Step
The horizontal component of the velocity (along the $\hat{x}$ direction) is constant, while at the highest point, the object stopped 'climbing' vertically so the vertical component (along $\hat{y}$ direction) is equal to zero there. So we have
$$\vec{v}_m=(4\text{ m/s })\hat{x}.$$
