The potential energy of a projectile at its highest point is (1/2)th the value of its initial kinetic energy. Therefore its angle of projection is ......
$ Hmax = { V_o^2 Sin^2 \theta_0 \over 2g } , U = mg Hmax = { mv_0^2 sin^2 \theta_0 \over 2} = {1 \over2} Ko $