What is the equation for
2x=18-y

 2x = 18 - y    is a perfectly good equation just as it stands.

If you were considering representing the equation as a graph, then
you might want to rearrange it, into a form that immediately reveals
some characteristics of the graph.  That would make your job easier. 

You'd want to rearrange it into 'standard' form for graphing, with 'y' on
one side of the equation and everything else on the other side, like this:

                                                 2x = 18 - y

Add 'y' to each side:              y + 2x = 18

Subtract  2x  from each side:       y  =  -2x + 18 

In this form, the equation shows that when it's graphed, the slope
of the line will be  -2, and the line will cross the y-axis at the point
where  y=18 .

Point is the only one not undefined