Answer
$y(t)=-2 cos(2\pi t)$
Work Step by Step
Since the motion started with a maximum negative displacement at $t=0$, we use a cosine model with
equation $y(t)=a\cdot cos(\omega t)$. It is given that $a=-2, \frac{2\pi}{\omega}=1, \omega=2\pi$
and we get the equation as $y(t)=-2 cos(2\pi t)$
