3x-y=0
y=3x
C(a,3a),r=|3a|
y=x
x-y=0
d^2=|a-3a|^2/2=2a^2
d^2+(2√7/2)^2=r^2
2a^2+7=(3a)^2
a=±1,r^2=9
(x±1)^2+(y±3)^2=9