[tex] m\angle ADC=m\angle ADB+m\angle BDC\\\\m\angle ADC=112^o,\ m\angle ADB=6x+7,\ m\angle BDC=12x-3\\\\\text{substitute}\\\\(6x+7)+(12x-3)=112\\\\(6x+12x)+(7-3)=112\\\\18x+4=112\ \ \ \ |-4\\\\18x=108\ \ \ \ |:18\\\\x=6\\\\m\angle ADB=6x+7\to m\angle ADB=(6(6)+7)^o=(36+7)^o=43^o [/tex]
