Looking at the graph
For x=0
y=3
the amplitude of the function is 3
the equation is of the form
y=AcosBx
where
A=3 -----> amplitude
Period ----> The Period goes from one peak to the next
Period=4pi
Period=2pi/B
so
2pi/B=4pi
B=2pi/4pi
B=1/2
therefore
[tex]y=3\cos \frac{x}{2}[/tex]
