RULE #1: you can add, subtract, multiply and divide by anything, as long as you do the same thing to both sides of the equals sign. ... RULE #2: to move or cancel a quantity or variable on one side of the equation, perform the "opposite" operation with it on both sides of the equation.
*NOTE* That's what I found, and I am NOT taking credit for it, because it is not my answer. I thought it could help you