Question 263483
Supplementary angles total 180.
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12x + 5 = 9x + 7
subtract 9x from both sides
3x + 5 = 7
subtract 5 from both sides
3x = 2
divide both sides by 3
x = 2/3
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so the supplementary angle must be 180 - 2/3 = 179 1/3.
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But that would be a very unusual case.
Did you leave something out?