Answer:
∠ A = 138° , ∠ C = 75°
Explanation:
ABCD has parallel bases and is a trapezium
each lower base angle is supplementary to the upper base angle on the same side.
∠ A + 42° = 180° ( subtract 42° from both sides )
∠ A = 138°
∠ B + ∠ C = 180 , that is
12x - 3 + 9x - 6 = 180
21x - 9 = 180 ( add 9 to both sides )
21x = 189 ( divide both sides by 21 )
x = 9
Then
∠ C = 9x - 6 = 9(9) - 6 = 81 - 6 = 75°