I have 3 scenarios for you, given that I'm not fully clear on the intent of your question:
1. y = -2x + 5 is already a function because it's a linear equation. Nothing is raised to a power here, so it's a straight line and the x's and y's already have a 1 to 1 match.
2. If a quadratic equation, which contains x^2 leads with a negative number, it just makes the parabola open downward.
3. The inverse of y = -2x + 5 would be:
x = -2y + 5. You only swap the variables, not the signs. You would still want this in y = mx + b form though, so you would just solve for y.
x = -2y + 5
Subtract 5 from both sides.
x - 5 = -2y
Divide by -2.
x/-2 - 5/-2 = -2y/-2
-1/2x + 5/2 = y or, better yet... y = -1/2x + 5/2.
Neither the domain or range would be resricted in any way because this is a linear equation.
Hopefully I hit your question with one of these outcomes!