(d) If $m$ is the mass, $r$ is the amplitude of oscillation, then maximum kinetic energy,
$K_{0} = \frac{1}{2}m\omega^{2}r^{2}\ \ or\ \ \ \ r = \left( \frac{2K_{0}}{m\omega^{2}} \right)^{\frac{1}{2}}$
The displacement equation can be
$y = r\sin{\omega t} = \left( \frac{2K_{0}}{m\omega} \right)^{\frac{1}{2}}\sin{\omega t}$