It can bother students that there is more than one unknown in an equation. The thing to remember is that all these variables are simply numbers. If it bothers you, plug in a random number for the variable and solve the equation. The steps you have to use to solve it in the form with the variable in it will be almost identical.
where any one of the three expressions in the last line do the trick. (Most would probably pick the last form, but it's entirely up to you.)