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Answer:
x = 12.75
∠C = 51.75°
Explanation:
The two marked angles are complementary, so ...
(5x -12) +(3x) = 90
8x = 102
x = 102/8
x = 12.75
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C = (5x -12)° = (5(12.75) -12)°
∠C = 51.75°
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Check
E = 3x = 38.25, then C+E = 51.75 +38.25 = 90 . . . . as it should