Greetings from Brasil...
In a trigonometric function of Cosine, we have:
F(X) = ±A ± B.COS(MX + N)
where
A = move the graph up or down
B = change the amplitude
M = change period
N = move the graph left or right without distorting it
According to the statement, we don't have A and N, so
F(X) = ± B.COS(MX)
According to the graph presented, we have:
B = amplitude = 2.5 = 5/2
period = π
detail that
Period = 2π/M
π = 2π/M ⇒ M = 2
As B = 5/2 and M = 2, then
F(X) = (5/2)COS(2X)
(I used the cosine function, because in X = 0 Y will have a non-zero value, as we can see in the graph)
