Question

How do you solve by substitution `x+y=0` and ##?

Answer 1

Substitute `(-x)` for each occurrence of `y` in whatever equation is supplied by "?"; simplify to find the value of `x`; then, equate `y` to the value of `(-x)`.

Explanation:

If `x+y = 0`
then
`color(white)("XXXX")` `y = -x`

(If, on the other hand, you did not mean for "?" to be a general indication of any non-specific, other equation, either fix the question or re-ask.