Answer:M∠DEC equals 123º.Step-by-step explanation:The sum of a triangle's three angles always equal 180º. The exterior angle, x, equals the two non-adjacent interior angles.180 - {(x - 45)+(x - 12)} = m∠DECm∠DEC + x = 180(x - 45) + (x - 12) = xSolving for x:(x - 45) + (x - 12) = xx - 45 + x - 12 = x Remove parenthesis2x - 57 = x Combine like terms2x = x + 57 Add 57 to both sidesx = 57 Subtract x from both sidesFinding m∠D:x - 45 = ?57 - 45 = 12º Finding m∠C:x - 12 = ?57 - 12 = 45º ** (Checking x: 12 + 45 = 57) **Finding m∠DEC:AC is a straight line, and because straight lines are equivalent to 180º, we subtract 57 from 180:180 - 57 = 123ºHope this helps,❤A.W.E.S.W.A.N.❤