Common Tangent to Two Non Intersecting Circles

Question

If the equations of two non intersecting circles are given in the general form i.e. $$x^2 + y^2 + 2gx +2fy + c =0$$ and $$x^2 + y^2 + 2g'x +2f'y + c' =0$$ , then how can I find common tangents on these circles. I am not able to figure out an approach for this. Also, what would be the lengths of these tangents