Answer
$y=-\ cos(\frac{3}{2} x)$
Work Step by Step
Step 1. As the curve is odd and decreases while passing $(0,0)$, we can write a general form $y=-A\ sin(kx)$
Step 2. We can identify the amplitude $A=1$.
Step 3. We can identify the period $p=\frac{4\pi}{3}$, thus $\frac{2\pi}{k}=\frac{4\pi}{3}$ and $k=\frac{3}{2}$
Step 4. The equation is $y=-\ cos(\frac{3}{2} x)$
