What form of equation produces a vertical line when graphed?

1. The definition states that a vertical line is the graph of an equation of the form x = a. 2. An equation of the form y = b instead produces a horizontal line. 3. An equation like y = ax or x + y = a still involves both variables changing together, which produces a slanted line, not a vertical one. _Source: OpenStax Elementary Algebra (CC BY 4.0), Ch 4 "Graphs", section 4.2 Graph Linear Equations in Two Variables_