Point m is x? +(y+ 1)？ =4

Center d should be (0,-1) instead of (0, 1).

The abscissa x=a of the center c is on the straight line y=2x-4:

y=2a-4

So: the coordinate of the center of the circle C is (a, 2a-4).

Distance to point D (0,-1): Cd2 = (a-0) 2+(2a-4+1) 2 = a2+(2a-3) 2.

So: CD = √ [A 2+(2A-3) 2]