Explanation:
The exterior angle is the sum of the opposite interior angles of the triangle.
Therefore, m<D +m<C = 118°
=> 8y+3 +3y+5 = 118°
=> 11y = 118° - 8°
=> 11 y = 110°
=> y = 10°
m<D = 8y+3
= 8(10)+3 = 83°
m<C= 3(10)+5 = 35°
m<DEC = 180° -118° = 62°