#### Answer

$s(t)=-2\cos 6\pi t$

#### Work Step by Step

The general equation regarding the oscillation of a weight attached to a spring is $s(t)=a\cos wt$.
Since the weight is first pulled down and then released, the equation becomes $s(t)=-a\cos wt$. Also, as the weight is pulled down 2 inches, the amplitude of the motion is 2. The equation therefore becomes $s(t)=-2\cos wt$.
Now we need to find $w$. Since the period is $\frac{1}{3}$, we can use the following formula to find $w$:
Period$=\frac{2\pi}{w}$
$1/3=\frac{2\pi}{w}$
$w=\frac{2\pi}{1/3}$
$w=6\pi$
Therefore, the model that gives the position of the weight at time $t$ is $s(t)=-2\cos 6\pi t$.