A fasor has 2 explicit variables (amplitude and phase) and 1 implicit(frequency).
So, a fasor is defined for a given frequency. In your case:
Y=4.5cos(2π*1600t)
It's clear that the frequency is 1600Hz, so its fasor will be defined for that frequency. Amplitude is 4.5. The phase has to be defined for a fixed reference point :
For example, suppose that in our system fasors are defined for a sine of frequency 1600Hz. The Y fasor will be amplitude 4.5 phase 90º , because sin(x+90º)=cos(x). If fasors are defined for a cosine, Y fasor will be Y=4.5 amplitude 0 phase.
I hope this helps. Best regards.