Finding the voltage supply (picture included)

[Josh225]

I have been having trouble calculating the value of the voltage supply. From what I have learned, to calculate the value, it is V= I x R. I have already calculated the value of R2 as the problem asks, but how do I find the voltage supply if I only know the resistane and the power being distributed to R2?

I tried using P= V^2 / R1 to try to find V, but it didn't work out for me:

V^2= 81 (18)= 1458
V= √1458
V= 38.18

The book says the answer is 27

 

[donpacino]

You are given the power dissipated in R2.

18 ohms is the value of R1, not R2. You need to use the value for R2 in your equation...

What did you get for R2

 

[Josh225]

The value of R2 was 9. Sorry, should have included that.

Oh wow... I see now. Thanks!