A linear equation is an equation with at least one linear term and any number of constants. No other types of terms may be included. Linear equations in one variable have the following form, where $a$ and $b$ are real numbers and $a\ne 0.$

Linear equations in two variables have the form below, where $a,$ $b,$ and $c$ are real numbers and $a\ne 0$ and $b\ne 0.$

Assume that either $a$ or $b$ is zero. In this case, the equation above becomes a linear equation in one variable. The graph of a linear equation is a line.