and if \({m_1} = {m_2}\) then the lines are parallel. The gradient of a line which is parallel to the \(y\)-axis cannot be determined from the above formulae. Just write the equation down!

Image caption, Parallel lines have the same gradient ... with the change in the 𝒙-coordinate (6 steps across the 𝒙-axis from 2 to 8 is 6) and the change in the 𝒚-coordinate (3 steps ...