As the picture below shows you can start the period anywhere you just have to start somewhere on the curve and end the next time that you see the curve at that height. In the case of the function y sin x the period is 2 p or 360 degrees. Locate two zeros that delimit a whole cycle or an integer number of cycles.
Then find the value for B in the equation of the function.
Is the vertical distance between the midline and one of the extremum points. Omega dfrac 2 pi lvert B rvert o B2p. The frequency is closely related to the period of the base trigonometric functions. As the picture below shows you can start the period anywhere you just have to start somewhere on the curve and end the next time that you see the curve at that height.