Well the right approach would be

 

In the whole of the path for every corresponding point in first half of trajectory there will be an corresponding point in the second half with the same hieght.

So at each pair of these points the vertical component of velocity will cancel out as the are equal and opposite but the horizantal component will add and will be equal to u*cos() each.

so for finding average velocity add vertical and horizontal components seprately.

The vertical components will cancel out and the horizontal component is constant so it will be the average velocity.