The question doesn't say what angle <ABD is supplementary to. Two angles are supplementary if their sum is 180 degrees.
If <ABD is supplementary to x, then <ABD=180-x.
But m<ABD=12x-12, therefore 12x-12=180-x, 13x=192, x=192/13.
If <ABD is supplementary to 12-x, then <ABD=180-(12-x)=168+x.
But m<ABD=12x-12, therefore 12x-12=168+x, 11x=180, x=180/11.