Answer:
7
Step-by-step explanation:
AB//CD (Opposite sides of parallelogram are parallel)
Angle A + Angle B = 180 (Sum of interior angles=180)
138 + 7x - 7 = 180
7x - 7 = 180 - 138
7x = 42 + 7
7x = 49
x = 49/7
x = 7
