I'm guessing d b are equal and a and c are equal or I don't think we would be able to solve so going on that
if angle d is 57 angle b must also be 57
A parallelegram has angles that add up to 360
So 360= 57+57+2c
Solve
360=114+2c
-114 both sides
246=2c
÷2 both sides
123=c