How Can You Decide Which Variable To Solve First When You Are Solving A Linear System By Substitution?

How can you decide which variable to solve first when you are solving a linear system by substitution?

 

For example the linear system is like this

3x + 4y = 12

y = x + 2

We will substitute y since its already given meaning that the first one that were going to get the value of one of the variables is x then after that we will now find the value of y

Well you could choose any variables to solve first, just solve one of the equations first for its one variable then solve for another