How much loss is there (in dB) in the unknown component?

Show your work clearly in PDF file,

A fiber system has 4 components between the transmitter and receiver. Three have losses of 3 and 5dB and 6dB respectively and the last has a loss that is unknown. The transmitter has a power output of 5mW. The received power is equal to 5nW.

How much loss is there (in dB) in the unknown component?