If x=-6 then y=54.
If x=-4 then y=51.
If x=-2 then y=48.
If x=0 then y=45.
If x=2 then y=42.
If x=4 then y=39.
If x=6 then y=36.
There are an infinite variety of pairs of numbers for 'x' and 'y'
that can make that equation true.
You can use that equation to draw a line on a graph. Then
EVERY point on the line is a solution to the equation.
'x' and 'y' don't have single values unless you have TWO equations.