x + 3 = 0 is not a function because it is a vertical line.
A vertical line has undefined slope because all points the line have the same x-coordinate.
A function is a relation in which no two ordered pairs have the same first component (x-values or input) and different second components (y-values or output).
Remember that a function can only take on one output for each input. Since a vertical line has the same x-coordinates for all values of y-coordinates, then it means that it is not a function.
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