What is an equation?
When you are solving an equation, you have to move everything except the missing number to one side.
Anything you do to one side, you must also do to the other.
For example: 6 + x = 15. To work out the value of x, first subtract 6 from the left side, 6 + x - 6 = x. Then subtract 6 from the right side too, 15 - 6 = 9. So you are left with x = 9.