A spring with a spring constant $k$ of 20 pounds per foot is loaded with a 10-pound weight and allowed to reach equilibrium. It is then displaced 1 foot downward and released. If the weight experiences a retarding force in pounds equal to four times the velocity at every point, find the equation of motion.

$y(t) =$
where $t$ is time (in seconds) and $y(t)$ is displacement (in feet).