7) \( x+y=0 \)

This equation represents a straight line in the Cartesian plane where the line passes through the origin (0,0) and has a slope of -1. In simpler terms, for any point (x, y) that satisfies this equation, if you know one coordinate, the other can be easily calculated. For example, if you take \( x=3 \), then \( y \) would be \( -3 \). In practical situations, this relationship can be useful in various fields like economics, physics, or engineering where one variable may inversely affect another. For instance, in budgeting, if you gain money in one area, you may need to reduce expenses in another area to keep your financial balance intact, thus obeying the same principle of balancing the equation!