The supplement of an angle is equal to 2 times the angle

Well if it is a supplement then that means that is has to equal 180 degrees. If one angle is two times the other and it equals to 180 you can make an equation out of this.
First you would do: 
x+2x=180

Then
3x=180

Last 
x=60

Therefore x will equal 60 degrees which means that angle 1 is 60 and angle 2 is 120

He gained 60,900 dollars for that land