Question

How do you solve the system `y=-x, y=-x-4`?

Answer 1

There are no "solutions"; the pair of equations represent two parallel lines.

Explanation:

If both `y=-x` and `y=-x-4` were true for some values of `x` and `y`
then
`y+x` would have to be equal to both `0` and `-4` for those values.