At the maximum height, all the kinetic energy will have been converted into potential energy:
mgh = 12\;\frac{1}{2}21mv2^22
v = y
gh = 12\;\frac{1}{2}21y2^22
h = y22g\;\frac{y^2}{2g}2gy2
Thus, the potential energy will be maximum at the height given by: y22g\;\frac{y^2}{2g}2gy2