Calculate transmitting power of the Transmitter
dB is defined in your first equation. All you need to do is to calculate what power ratio corresponds to 20 dB. If you substitute dB = 20, and solve for P1/P0, you get P1/P0 = 10^2 = 100.  Therefore P1 = 100P0.  If dB = 3, then substitute that value into the first equation and again solve for P1/P0.  You get P1/P0 = 10^.3 = 1.995 which is usually rounded to 2.  So 3 dB corresponds to a power ratio of 2. Therefore P1/P0 = 2 and thus P1 = 2P0.