[tex]\bf \begin{array}{lllll}
&x_1&y_1&x_2&y_2\\
% (a,b)
&({{ 8}}\quad ,&{{ -2}})\quad
% (c,d)
&({{ r}}\quad ,&{{ -6}})
\end{array}
\\\\\\
% slope = m
slope = {{ m}}= \cfrac{rise}{run} \implies
\cfrac{{{ y_2}}-{{ y_1}}}{{{ x_2}}-{{ x_1}}}\implies \cfrac{-6-(-2)}{r-8}=-4
\\\\\\
\cfrac{-6+2}{r-8}=-4\implies -4=-4(r-8)\implies \cfrac{-4}{-4}=r-8\\\\\\ 1+8=r[/tex]
