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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