am i correct?

You forgot to include the sin function...

2.5 sin (1318 pi x)

Frequency  = 1 / period

So

Period = 1 /Frequency =  1 / 659

We have the form

y = A sin (B x)

A = the amplitude = 2.5

B =  [ 2pi ] / period  =   2pi / (1/659)   =   1318 pi

So....as the guest found....the function is

y = 2.5sin (1318 pi * x )

like so?

http://prntscr.com/mu99ml

thank you