A(2,1)
y+2x=0
C(a,-2a)
x-y-1=0
r^2=d^2=CA^2
|a+2a-1|^2/2=(a-2)^2+(-2a-1)^2
a=-3
C(-3,6),r^2=50
(x+3)^2+(y-6)^2=50