The fromula of a slope:
[tex]m=\dfrac{y_2-y_1}{x_2-x_1}[/tex]
We have
[tex](2,\ 5);\ (7,\ r),\ m=\dfrac{1}{3}[/tex]
Substitute:
[tex]\dfrac{r-5}{7-2}=\dfrac{1}{3}\\\\\dfrac{r-5}{5}=\dfrac{1}{3}\qquad|\text{cross multiply}\\\\3(r-5)=(5)(1)\qquad|\text{use distributive property}\\\\3r-15=5\qquad|\text{add 15 to both sides}\\\\3r=20\qquad|\text{divide both sides by 3}\\\\r=\dfrac{20}{3}[/tex]
