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