Not quite. The ball strikes the ground at an actual velocity of 15m/s at an angle of 45 degs.
The horizontal component of this velocity is 10.6 m/s (=15 cos 45)
As the horizontal component doesn't change and the object was initially projected horizontally, it's initial horizontal speed (and speed of projection) must have been 10.6 m/s
The initial vertical component was zero but this has increased as a result of gravity and is now actually =15 sin 45