Answer:
y = 120°
Step-by-step explanation:
Parallelograms are unique figures that have a few important properties. One property they have is that the opposite angles are congruent (equal in measure), in this case ∠A = ∠C. Another property that they have is that Consecutive angles are supplementary (add up to 180°), so ∠C + ∠D = 180°. Knowing these two properties, we can solve for 'y' be setting up two equations, one to solve for 'x' and the other to solve for 'y'. We must first solve for 'x':
∠C + ∠D = 180 or 3x + x + 8 = 180
Combine like terms: 4x + 8 = 180
Inverse operations: 4x = 172
Solve: x = 43
Next, solve for y: ∠C = ∠A or 3(43) = y + 9
Inverse operations: 129 - 9 = y + 9 - 9
Solve: y = 120
