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Answer :

Answer:

x = 12

Step-by-step explanation:

Complementary angles add up to have a sum of 90 degrees. We can create an equation and solve for x.

3x + 6x - 18 = 90

~Simplify

9x - 18 = 90

~Add 18 to both sides

9x = 108

~Divide 9 to both sides

x = 12

Best of Luck!