Good morning
21 + 3q < 3(13 - q)
21+ 3q < 39 - 3q Here it was -3q and it was on the left side. However when you are switching it to another terms that looks like it you must change the sign as well.
3q + 3q < 39 - 21 Here I bring the common terms together.
6q < 18
Divide both sides by 6
6q/6 < 18/6
q < 3
