Note that the of a line is undefined only when the line has the equation ##x=c## then proceed as usual.
Almost line in the ##xy## plane can be expressed as ##y=mx+b## where ##m## is the line’s slope and ##b## is its ##y##-intercept. The exception is when the slope of the line is undefined that is in the case of a vertical line.
A vertical line has an equation of the form ##x=c## for some constant ##c##. If an inequality has such a line as a boundary we simply pick whichever side of that line contains points satisfying the inequality and shade that just as we would with a line that has a defined slope.
For example here is the graph of ##x<3##: In a system of inequalities we just shade as normal for each inequality and then keep wherever all of the shadings overlap. If we have ##{(x > -2) (x <= 2) (y > 2x-1):}##
we would graph the three lines generated by equalities (remembering to used dashed lines for ##>## or ##<## and solid for ##>=## or ##<=##) and then keep the portion in which all three shaded areas overlap giving us this: