It isn't. is simply a binomial expression in one variable. Now if you make it equal to something, such as zero, THEN you have an equation whose graph and/or solution set is a straight line, and in this particular case, a vertical line.
The reason it is a vertical line is that the set of points that comprise the solution set of the equation all have an ordinate, or -coordinate, of the value 4. All points with an -coordinate of 4 must lie on a vertical line. Go ahead and try to find a point with an -coordinate of 4 that is NOT on the vertical line .
John
My calculator said it, I believe it, that settles it
