You can put this solution on YOUR website! By definition, a pair of supplementary angles are angles that add up to 180º. Remember that.
So let's call our two angles A and B.
Let A+B = 180
And let A = 18+B ---> A-B = 18
Then we have a system of equations.
A+B = 180
A-B = 18
Notice if we added these two equations we would eliminate B. That's good.
2A = 198
A = 99
Sub 99 back in for A. 99 + B = 180 Then B = 81.
Our two angles are 99 and 81.