So I like to think of questions like these reversing everything and unravelling the Answer
The first step you'd do in this question is add the two to both sides since it doesn't have a number attached to it
This will minus the -2 from the left hand side and add it to the right hand side so now your question will look like this:
-3r=21
Got it?
Okay
so now that's done divide the -3 on both sides so 21 divided by -3
so now your Answer will look like this:
r=-7
hehehe sorry about that no wonder I'm generally bad at Maths, I get confused X3