Intermediate Algebra/Writing Linear Equations

Linear equations have two variables: $$x$$ and $$y$$.

Examples
$$ y=5+7x $$

$$ y=2-4x+4 $$

$$ y=7/2-4x $$

If one were doing a walkathon and wanted to raise $5 for every mile walked, an equation for the total money raised, $$y$$ as a function of the number of miles walked, $$x$$ would be $$y = 5x$$. Would donors, however, promise to not only pay $5 per mile but give an additional $30 just for taking part, the equation would be $$y=5x + 30$$. 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.