For Ax+By=C, we have y=-3x. We want both the y and x variable on the same side, and that leads us to either subtract y from both sides or add 3x to both sides. You can do either, but I personally prefer the latter, so we have 3x+y=0. We have A=3 (since that is the coefficient for x) and B=1 (since 1*y=y). Lastly, C=0
