Linear equations have two variables: ${\displaystyle x}$ and ${\displaystyle y}$.

Examples

${\displaystyle y=5+7x}$

${\displaystyle y=2-4x+4}$

${\displaystyle y=7/2-4x}$

If one were doing a walkathon and wanted to raise $5 for every mile walked, an equation for the total money raised, ${\displaystyle y}$ as a function of the number of miles walked, ${\displaystyle x}$ would be ${\displaystyle y=5x}$. Would donors, however, promise to not only pay $5 per mile but give an additional $30 just for taking part, the equation would be ${\displaystyle y=5x+30}$. Note that this example is limited as negative miles don't make sense, but the line defined by the equation is infinite in either direction.