Kastamonou..study again the chapter of the Analytic Geometry refering to the tangent lines to a given circle.:
I remind you that if x^2+y^2+ax+by+c=0 is the equation of a circle then the legth of the tangent segment to the circle drawn from an external point P(h,k) is :
sqrt(h^2+k^2+ah+bk+c) .In your case this is equal to 4 therefore substitute in the formula and get the right answer which is -1.