Supplementary angles are two angles that add up to 180°
The unknown angle is known as x
The second angle, being 18° more than the unknown angle, is x+18
So, the expression can be written as x+x+18=180
Subtract 18 from both sides.
x+x=162
Combine like terms
2x=162
Divide by 2 on both sides
162/2=81
x=81
So, one of the angles is 81°
The other is 99° ( because 81+18)