The general form of such an equation will be
[tex]f(x)=A\cos\left(\dfrac{2\pi(x-B)}{C} \right)+D[/tex]
where
A = amplitude (3)
B = horizontal shift to the right (-π)
C = period (4π)
D = vertical offset (0)
Your equation will be ...
[tex]f(x)=3\cos\left(\dfrac{x+\pi}{2}\right)[/tex]
