Answer:
x = 7, arc DE = 198°
Step-by-step explanation:
Inscribed angles from the same arc are congruent.
∠ DCE and ∠ DWE are inscribed angles from the same arc DE , thus
12x + 15 = 3x + 78 ( subtract 3x from both sides )
9x + 15 = 78 ( subtract 15 from both sides )
9x = 63 ( divide both sides by 9 )
x = 7
Thus
∠ DCE = 12x + 15 = 12(7) + 15 = 84 + 15 = 99°
The inscribed angle DCE is half the measure of its intercepted arc, thus
arc DE = 2 × 99° = 198°
