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In your first version, solve views y as a known parameter (that Sage does not assume up front to be -2) and not a variable. So, the system doesn't have a solution.

In your second version, solve views y as a variable and solves for it.

In your first version, solve views y as a known parameter (that Sage does not assume up front to be -2) $-2$) and not a variable. So, the system doesn't have a solution.

In your second version, solve views y as a variable and solves for it.