If then:

Thus, and

If then:

Thus, and

Thus, we have that this line must contains the points

Thus, the Symettric Equation is:

Note, this is a zero in the denominator of the first fraction, it is a useful notation I saw in a Russian Textbook* which simply means constantly, so it is just a shorthand notation.

*)If you are interested the Textbook was written by Perelman! Not Perelman you think it is but his grandfather!