we can always get the slope of a line by simply picking two points off of it, say in this case hmmmm let's use (20, 20) and (10, 0), which are the ones you seem to have used.
[tex]\bf (\stackrel{x_1}{20}~,~\stackrel{y_1}{20})\qquad (\stackrel{x_2}{10}~,~\stackrel{y_2}{0}) \\\\\\ slope = m\implies \cfrac{\stackrel{rise}{ y_2- y_1}}{\stackrel{run}{ x_2- x_1}}\implies \cfrac{0-20}{10-20}\implies \cfrac{-20}{-10}\implies 2[/tex]
