geometry
posted by lucky bee .
Each of the three circles have a radius of 10. The line OA contains the center of the three circles, and the line OB is tangent to the righthand circle. Find the length of segment CD

There are a lot of loose ends in this explanation. You give no explanation of the relative positions of the circles, where A,B,C,D are, or even O.
All you say is there are 3 circles (somewhere) and that their "center" is on line OA. (by the way, what is the "center" of 3 circles?)