Hey there!
First we need to add by [tex]21[/tex] on each of your sides
Like: [tex]3r - 21 + 21 \\ \\ -36 +21 [/tex]
This gives us: [tex]3r = 15[/tex]
Now we have to [tex]divide [/tex] by [tex]3[/tex] on each sides
Like: [tex] \frac{3r}{3} \\ \\ = \frac{-15}{3} [/tex]
[tex]Answer: r = -5[/tex]
Good luck on your assignment and enjoy your day!
~[tex]MeIsKaitlyn:)[/tex]
