ABCDEFGHIJKLMNOPQRSTUVWXYZ Definition of Equation of a Straight Line more ... An equation that defines a straight line. In this example the equation is y = 2x + 1 It could also be written y − 2x = 1 or many other ways. Try some different lines below: ../geometry/images/geom-line-equn.js?mode=equn Equation of a Straight Line