The capital pi confused me a bit, thought it was a stray character typo, should be lowercase pi.
Also don't make the same (reading) mistake I did and think pi*y is the volume of the sphere instead of the cylinder!
Trick is to draw the radius of the sphere and find a right triangle, and use what you know about right triangles.
Actually i have already worked backwards and i realise that r^2 = 25-1/4h^2.
And i tried to form a right angle triangle with r and h/2 and by working backwards using pythogras theorem, i know that the hypontenus is 25. However, I just simply dont get it why is the hypontenus of this right angle triangle 25.