If the circles x^2 + y^2 + 12x + 6y - 19 = 0 and x^2 + y^2 - 4x - 6y = k are tangent to each other, find the possible values of k.

How can I know whether the unknown circle is inside or outside the circle x^2 + y^2 + 12x + 6y - 19 = 0?

or without knowing this, can I still solve the problem?