Please explain and walk through problem. Thanks

context: "Solve each system of linear equations by substitution."

Solving a system by substitution means that you use one equation (in this case, the second) to express one variable in terms of the other (in this case, in the second equation, we express in terms of .

So, we can substitute this expression for in the first equation, we have

But since we know that

We have

We can now solve this equation as usual, since it involves alone:

And once we know that value for , we can compute :