Answer
$y=-10 \ \cos(\dfrac{2\pi t}{3})$
Work Step by Step
The object starts at the vertical position $y=-a$, and moves in the upward direction.
The frequency is given by:
$\omega=\dfrac{2\pi}{T}$
So, the function has the form (simple harmonic motion):
$y=-a\cdot \cos(\omega t)=-a\cdot \cos(\dfrac{2\pi}{T} t)$.
Here,
$y=-10 \times \cos (\dfrac{2\pi}{3} \times t)=-10 \ \cos(\dfrac{2\pi t}{3})$
