Answer
$y=- 6 \ \sin( 2t)$
Work Step by Step
The object starts at the vertical position $y=0$, and moves in the downward direction.
The frequency is given by:
$\omega=\dfrac{2\pi}{T}$
So, the function has the form (simple harmonic motion):
$y=-a \ \sin(\omega t)=-a \ \sin(\dfrac{2\pi}{T} t)$.
Here,
$y=- 6 \times \sin (\dfrac{2\pi}{\pi} \times t)= - 6 \ \sin( 2t)$
