we can always find the slope of any line by simply using two points on the line, say let's use (3,4) and (-1,2)
[tex]\bf (\stackrel{x_1}{3}~,~\stackrel{y_1}{4})\qquad (\stackrel{x_2}{-1}~,~\stackrel{y_2}{2}) \\\\\\ slope = m\implies \cfrac{\stackrel{rise}{ y_2- y_1}}{\stackrel{run}{ x_2- x_1}}\implies \cfrac{2-4}{-1-3}\implies \cfrac{-2}{-4}\implies \stackrel{\stackrel{\textit{common difference}}{\downarrow }}{\cfrac{1}{2}}[/tex]
