x2+y2+ax+2ay+c=0
2g2−c=24a2−c=22
⇒4a2−c=2...(1)
2f2−c=2a2−c=25
⇒a2−c=5...(2)
(1)&(2)
43a2=3⇒a=−2(a<0)
∴c=−1
Circle ⇒x2+y2−2x−4y−1=0
⇒(x−1)2+(y−2)2=6
Given x+2y=0⇒m=−21
mtangent=2
Equation of tangent ⇒(y−2)=2(x−1)±61+4
⇒2x−y±30=0
Perpendicular distance from (0,0)=∣4+1±30∣=6