Can someone please explain to me how to start off this question?

A uniform semicircular lamina has mass M. A is the midpoint of the diameter and B is on the circumference at the other end of the axis of symetry. A particle of mass m is attached to the lamina at B. The centre of mass of the loaded lamina is at the midpoint of AB. Find in terms of pie, the ration M:m .

Some one please help quickly