When rewriting equations it's important to make the the same operation on both sides to ensure that the equality still holds. So, if one side is divided by a number, the other side must be divided by the same number. In the equation $3x=12$ both sides is divided by $3$ to solve for $x.$ Remember that division by zero is undefined, so, for example, by dividing both sides by $x$ it is assumed that $x$ cannot be zero.