you can multiply either the top or bottom equation by -1 so lets say you apply it to the bottom equation it’ll look like this:
-9x + 9y = 18
6x - 9y = 12
then the +9y and -9y will cancel out
-9x = 18
6x =12
then you add the two equations together
-3x = 30
divide both sides by -3
x = -10
