The general linear equation is usually written y=mx+b, where m is the slope=(difference between coord)/(difference between x coord); BUT if the difference between the x coord is 0 then the line is vertical (the slope is infinite meaning 90 degrees), because, no matter what the value of y is, x=-1, so that's the equation of the vertical line: x=-1. The more general linear equation is ax+cy+d=0 so in this case a=1, c=0 and d=1.