A $1\mathrm{.}5$ kg object oscillates at the end of a vertically hanging light spring once every $0\mathrm{.}40$ s$.$ Write down the equation giving its position y $(+$ upward$)$ as a function of time t$.$ Assume the object started by being compressed $18$ cm from the equilibrium position $($where y $=0),$ and released. Note: the equilibrium position is defined here as that location of the mass at rest when$$it is freely hung from the spring, not the unstretched position of the$$spring with no mass. How long will it take to get to the equilibrium position for the first time?