What is the power gain in dB for a voltage amplifier with a ratio of 200 between output voltage and input voltage?

[MissP.25_5]

An amplitude of output voltage Vo of a voltage amplifier was compared to the amplitude of input voltage Vi. The ratio Vo/Vi was 200.

Find the power gain in dB.

Can someone explain to me how to find this? The solution is given as below, but I don't understand how can Po/Pi = 200^2

 

[rude man]

Power goes as the square of voltage, assuming equal resistances: P = V^2/R.
The solution is correct.

 

[MissP.25_5]

Power goes as the square of voltage, assuming equal resistances: P = V^2/R.
The solution is correct.

Ah, I got it. There's another subquestion that I really don't understand. It says to find the output voltage when Vi is -6 dBV. I don't understand the solution. If Vo is 200 times larger than Vi, then how come Vo = 40 dB?

 

[MissP.25_5]

Disregard my question. I have understood it :)
The solution seems to be written in a descending order.