We start from Y = Sin(X) , where the amplitude is 1 and the period is 2(pie)
In this problem, we can see the amplitude is
[tex]A=\frac{2-(-2)}{2}=\frac{4}{2}=2[/tex]and the period is
[tex]\frac{15}{2}\pi-\frac{3}{2}\pi=6\pi[/tex]When the period is 6*pie, the value of X in Sin(X) changes to X/3. Therefore, We obtain Sin(x/3)
In conclusion, the equation for this graph is
[tex]y=2\sin (\frac{x}{3})[/tex]