[tex]\angle A\cong\angle C\ \text{thereofre the triangle ABC is an isosceles triangle}\\\text{Therefore we have the equation:}\\\\4x=2x+6\ \ \ \ \ |subtract\ 2x\ from\ both\ sides\\2x=6\ \ \ \ \ |divide\ both\ sides\ by\ 2\\\boxed{x=3}\\\\AB=4x\to AB=4\cdot3=12\\\\ANSWER:\ D.\ 12[/tex]
