A vertical spring with spring stiffness constant $305$ N$$m oscillates with an amplitude of $28\mathrm{.}0$cm when $0\mathrm{.}235$kg hangs from it$.$ The mass passes through the equilibrium point $($y $=0)$ with positive velocity at t $=0.($a$)$ What equation describes this motion as a function of time? $($b$)$ At what times will the spring be longest and shortest?