The shift is the quantity subtracted from the variable inside cos. It seems that the shift is divided by the coefficient of t, giving 3π/10. I don't know why this is, but I just varified it with a graphing program and it's true.

]]>Now I know that the amplitude is 15 (obviously)

but how do i work out the frequency and phase.???

ironicly i have the answers which are that the period (T) = 2pi/5 and the phase shift is 3pi/10 compared to y=15cos5t

but how do u do it?

please help me