How to decide whether the relation x+y=25 defines a function?

It is a FUNCTION if we use the vertical line and the graph of the given relation only intersects at one specific point. Otherwise, it is not.
Kindly see the graph of ${\displaystyle x+y=25}$
graph{(x+y-25)(y+10000x-15*10000)=0[-50,50,-25,25]}
How to test using the vertical line test
Example: the graph of ${\displaystyle y^2-4y=2x}$ is NOT A FUNCTION because, using a vertical line will intersect in more than one point.
graph{(y^2-4y-2x)(y+10000x-15*10000)=0[-20,20,-10,10]}
Example: the graph of ${\displaystyle 2x^2-5x=3y}$ is a FUNCTION because, using a vertical line will intersect in only one point.
graph{(2x^2-5x-3y)(y+10000x-2*10000)=0[-20,20,-10,10]}