needing a formula

i need to find a way to solve this type of problem. we have been given a transmitter that is giveing a weak single strenght of only 7 watts it should be giving 28 watts and our reciever is getting a single of 5 micro volts. what will it recieve if we fix the transmitter to produce the full 28 watts. the answer is 10 micro volts but the only formula i have to work with is for db. db=20 log vo/vi the problem is that had this not been a multiple choice question i would noy have had the vo (voltage out) to start with. got to be a better way. if you can help my email is notfromnot@mailcity.com thanks jon