I know this formula. the problem is to find the Gain.
As far as I know the link budget is:
Pr = Pt+Gt-Lfs-Lm+Gr
Pr, Pt – power of the receiver and transmitter (how to know them??)
Gr, Gt – gains
Lfs – free space loss
Lm – other losses (which one? how to find them?)
and I still do not know how in this problem I should use the size of the covered area (within the 3dB of transmitted power)