cb^2 + ab^2 = ca^2 pythagorean theorem
cb^2 +6^2 = 12^2
cb^2 +36 = 144
cb^2 = 108
cb = sqrt (108)
sin D = opp /hyp
sin D =bc/cd
sin 55 = sqrt(108)/cd
cd = sqrt(108)/sin 55
cd=12.687
cd = 12.7 to three significant figures
