i don't quite understand how to get the length of dm, can someone help?
I'll give you a few hints:
1) Draw an altitude h from D to a point F on line segment AM. h = 2r
2) Find the lengths of lines AF and FM
3) Use the Pythagorean theorem to find the length of DM.
(btw., ignore those suggested answers; the length of DM is totally different from those numbers.)