Calculating Max distance from TX to RX that satifies SNR
The only problem is I have not been able to find one single example
  that illustrates about how to find Prx with my given parameters


The generally accepted figure for minimum rx power is based on bandwidth thus: -

Prx = (-154 + 10log\$_{10}\$(bit rate)) dBm

Formula taken from page 15 of this document (essentials of radio wave propagation).

So, for a bandwidth of 20 MHz, Prx(min) = -81 dBm

In the above I have assumed that the channel BW is equal to the bit rate.

You then have to take account of the 5 dB noise figure and the SNR.

See also this wiki page for a different version of the formula

Hope this helps.