This is a parallelogram which means the two opposites sides are equal. Then, we can solve the problem as shown below:
mAB = m DC
9x -14 = 3x + 4
Transpose and combine same term:
9x - 3x =4 +14
6x= 18
x=3
Solving for DC:
DC = 3X + 4
DC = 3(3) + 4
DC = 13 units.
The answer is 13.