Rule denotes to equation
[tex]\\ \rm\rightarrowtail m=\dfrac{3-1}{12-10}=\dfrac{2}{2}=1[/tex]
Equation in point slope form
[tex]\\ \rm\rightarrowtail y-1=x-10[/tex]
[tex]\\ \rm\rightarrowtail y=x-9[/tex]
So
[tex]\\ \rm\rightarrowtail n=20-9=11[/tex]
