first find the radius of the circle using the centre  co-ordinates and the equation of the tangent which is given.........  Then using the radius and the centre find out the equation of the circle.....  

 Then by using this... ( in circle equation)

x.x₁ + y.y₁ + g( x + x₁) + f( y+y₁) + c = 0  

put (x₁, y₁) as (0.0) as the tangent passes through origin.... 

is this right?  sorry can't post the complete solution..