all the opposite angles are equal in a parallelogram:
so AB=DB shows it is an isosceles triangle:
so the two angles on the sides are congruent,
let x to be one of the angles:
2x+28=180
2x=152
x=76=∠A
∠A is opposite of ∠DCB so ∠DCB is 76° because of the parallelogram properties
