I will digress from what we've spoken of but I wanted to get this out there:
A set of simultaneous equations, also known as a system of equations or an equation system, is a finite set of equations for which common solutions are sought.
i.e. there exists solutions to your set of equations given that they're related.
So there are three primary methods to solving systems of equations that is elimination, substitution, and graphing, you'll need to know how to do all three although you wouldn't really need graphing as you probably aren't given graphing paper, and it's the most time consuming assuming you don't have a graphing calculator or some other graphing device.
So when it comes to substitution just like elimination it doesn't matter what indeterminate you're solving, or what equation you're using, you'll just have to do the other one as if you done the preceding (this sentence probably doesn't make sense, I don't know how else to word it).
So in our case we decided to solve for x using the second equation, that means we have to substitute our x from our second equation into our first, then we solve for the single variable.