Respuesta :

Answer:

m∠DEC = 123°

Step-by-step explanation:

First we need to solve for x the equation: x = (x - 45) + (x-12)

x = (x - 45) + (x-12) ⇔ x = 2x -57 ⇔ 0 = x - 57 ⇔ x = 57

since m∠DEC = 180 - x  then   m∠DEC = 180 - 57 = 123

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