Answer
$y=-2\ sin(\frac{\pi}{2}x)$
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 $A=2$.
Step 3. We can identify the period $p=4$, thus $\frac{2\pi}{k}=4$ and $k=\frac{\pi}{2}$
Step 4. The equation is $y=-2\ sin(\frac{\pi}{2}x)$
