The frequency [tex] f [/tex] is defined as
[tex] f = \dfrac{1}{T} [/tex]
where [tex] T [/tex] is the period, i.e. the time between two maximum/minimum points.
In your example, you can see that a maximum is reached when [tex] x = 0 [/tex], and the next one at [tex] x = \pi [/tex]. This means that the period is
[tex] T = \pi-0 = \pi [/tex]
So, the frequency is
[tex] f = \dfrac{1}{\pi} [/tex]
