What is the mathematical equation to calculate the RSS or RSSI at a receiver, receiving a signal via single reflection ?
The reflecting surface will cause a loss of received signal due to the angle of incidence and refraction from, and absorption within, the reflector. 

Your problem sheet states that the reflection coefficient is known so you apply the Friis equation and treat the reflection as a loss based upon the reflection coefficient.

If you know the dielectric properties of the transmission mediums, you could also calculate the multipath effect of the signals.