what is the line obtained from the equation of 2 intersecting circles?

Hi, I am struggling with this problem (Angry). I do not know the properties of circles so well, so I gave the variables some values and drew, but I still don't get anything :-/. It would also be great if you could tell me how you know (what rule, etc.) you have used to answer the question. Thanks! :)

The problem says:

Let , be two intersecting circles. Deviding the first equation by a and the second by b and subtracting, we obtain the equation of a line:

.

What is this line?