Because this is a parallelogram, both the bottom and top bases are equal.
2c - 6 = 20
add 6 to both sides
2c = 26
divide both sides by 2 to isolate the variable
c = 13
The interior angles are alternating interior angles.
4b = b + 120
subtract b from both sides
3b = 120
divide both sides by 3 to isolate the variable
b = 40