Question

When you solve the equation x + 9 = 15 , how do you know what number to subtract from both sides? Why is it important to subtract the same number from both sides of the equation?

Answer 1

you subtract the *constant* from both sides. It is important to subtract the same number so that you maintain *equality*

Answer 2

Well, you want to get x alone. That is why you would want to subtract 9 from both sides.