Answer:
12 cm
Step-by-step explanation:
AB and DC are parallel to each other, so they are the same length. Set their lengths equal to each other and solve for x
4x = 7x - 9
4x - 7x = 7x - 7x -9
-3x = -9
-3x/-3 = -9/-3
x = 3
Now solve for AB.
AB = 4x
AB = 4(3)
AB = 12
x = -3
i explained it in the picture
