Answer:
m∠C = 75°
Explanation:
Theorem:
The consecutive angles of a parallelogram are supplementary (sum to 180°).
⇒ m∠D + m∠E = 180°
⇒ (7x + 21) + (15 + 5x) = 180
⇒ 7x + 21 + 15 + 5x = 180
⇒ 12x + 36 = 180
⇒ 12x = 144
⇒ x = 12
Theorem:
Opposite angles of a parallelogram are equal.
⇒ m∠C = m∠E
⇒ m∠C = 15 + 5x
Substituting found value of x:
⇒ m∠C = 15 + 5(12)
⇒ m∠C = 75°