ANSWER
r = -4
EXPLANATION
The formula for a slope of a line that passes through points (x1, y1) and (x2, y2) is:
[tex]m=\frac{y_1-y_2}{x_1-x_2}[/tex]
The order of the points doesn't matter as long as you write both coordinates of the same point first (or second). The formula above is the same as:
[tex]m=\frac{y_2-y_1}{x_2-x_1}[/tex]
With this in mind, we can find r with either of the forms of the formula:
[tex]-3=\frac{-10-r}{6-4}=\frac{r-(-10)}{4-6}[/tex]
Using the first one:
[tex]\begin{gathered} -3=\frac{-10-r}{2} \\ -3\cdot2=-10-r \\ -6=-10-r \\ 10-6=-r \\ 4=-r \\ r=-4 \end{gathered}[/tex]
