Combine -7x+12 and -8x+48 to get -15x+60. Since BDA is a straight angle put the above equation to get -15x+60=180. Subtract 60 from both sides to get -15x=120. Divide -15 both sides to get -8 as your x value. Plug in -8 to BDC, since both negatives are being multiplied, it would turn to a positive number which is 56, add 12 to get 68 as your final result for BDC.
