hi guys. this question make my head spin can someone clear it out for me?

Two discs, centres A and B, are at rest. Disc B lies on a horizontal wall. Its radius is 10 cm. The discs are in contact with each other at point P, and are each in contact with a vertical wall. The line through AB makes an angle of 70 with the horizontal wall. Disc B now begins to roll away from the vertical wall. This allows disc A to fall vertically. How far has B rolled when AB is inclined at 50 to the horizontal?