plot[{20*log_10(100)+60,20*log_10(80)-20x+60,20*log_10(400)-40x+60}, {x,-1,3}]

Uh oh! Wolfram|Alpha doesn't run without JavaScript.

Please enable JavaScript. If you don't know how, you can find instructionshere.Once you've done that, refresh this page to start using Wolfram|Alpha.