(a) Let $h$ be the desired height of liquid in cylinder for which the force on the bottom and sides of the vessel is equal
Force on bottom $= \rho\ gh \times \pi R^{2}$
Force on the walls of vessel
$= \rho\ g\ (h/2) \times 2\pi R\ h$
$= \rho\ g\ \pi h^{2}R = \rho\ g\ h\ \pi\ Rh$ or $R = h$