The problem is in the picture.we know the alpha angle and the constant value of velocity v.then find the value of speed "u" when the thread makes angle alpha with horizontal.

u = v / cos α

Explanation:

Sorry for my first try. I think I figured it out this time.

When you pull on the rope the block wants to go in the direction v. But it has to stay on the plane. So it will also move in the direction w. (see my picture) The result is that the block moved in the direction u, which is the vectorial sum of v and w.

All three vectors form a right triangle, where v = u * cos α. If you solve for u, you will get: u = v / cos α