Consider, in turn, linear equations in one, two, and three variables. A linear equation in one variable x is an equation of the form ax + b = 0. If a ≠ 0, this has the solution x = −b/a. A linear equation in two variables x and y is an equation of the form ax + by + c = 0. If a and b are not both zero, and x and y are taken as Cartesian coordinates in the plane, the equation is that of a straight line. A linear equation in three variables x, y, and z has the form ax + by + cz + d = 0. If a, b, and c are not all zero, and x, y, and z are taken as Cartesian coordinates in 3‐dimensional space, the equation is that of a plane. See also simultaneous linear equations.