Answer
[tex]\begin{gathered} Let\text{ }m\text{ }be\text{ }the\text{ }slope\text{ }of\text{ }a\text{ }line\text{ }joining\text{ }two\text{ }points\text{ }(x_{1,}y_1)\text{ }and\text{ }(x_{2,}y_2) \\ then\text{ }m=\frac{y_2-y_1}{x_2-x_1} \end{gathered}[/tex][tex]\begin{gathered} Let\text{ }(x_{1,}y_1)=(6_,r)\text{ }and\text{ }(x_{2,}y_2)=(18_,15),m=\frac{3}{2} \\ Using\text{ }the\text{ }above\text{ }formula \\ \frac{3}{2}=\frac{15-r}{18-6} \\ \frac{3}{2}=\frac{15-r}{12} \\ Cross\text{ }multiply \\ 2(15-r)=3(12) \\ 30-2r=36 \\ -2r=36-30 \\ -2r=6 \\ \frac{-2r}{-2}=\frac{6}{-2} \\ r=-3 \end{gathered}[/tex]
Therefore, the missing Co-ordinate r = -3
