Not entirely sure what you are referring to, it is something like a general formula:

?

Here, we have:

We deduce the value for b from the number of 'loops' of the graph - and as you say, this can be calculated through knowing the period. is periodic in that it repeats itself every radians. This means that:

has one full 'loop' between and Here, there are two full loops, so

The way your text describes it: