Block will experience maximum force due to friction when it is at maximum height, h at this height, if slope of the tangent is tanθ, then θ= Angle of repose. ∴tanθ=
dy
dx
=
d
dk
(
x2
4
)=
x
2
=0.5⇒x=1 and therefore y=0.25m=25cm (Assuming that x&y in the equation are given in meter)