Two Angles are Supplementary if they add up to 180 degrees.
So (9a-9) + (9a +18) should add up to 180 degrees
9a-9 + 9a+18= 180
group similar factors together
9a+9a =180-9
18a = 171
divide by 18 both sides
a = 171/18
a = 9.5
then the first angle is 9a-9 = 9x9.5 -9= 85.5-9= 76.5 degress
then the other angle would be 180-76.5 = 103.5 degrees
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