Answer
$y=-50cos(8\pi t)$
Work Step by Step
Given $f=4,2a=100$, since the mass is at its lowest point when t = 0,
we should use a $-cos$ function as $y=-a\cdot cos(\omega t)$ with $\omega=2\pi f=8\pi$
The motion function becomes $y=-50cos(8\pi t)$
