In order to substitute, you'd just find the same variable in both equations, isolate the variable that you want to substitute something in for, and then put it in place of the variable in the other equation. Let's break this down:
Between the first and second equation, both have A and B, while the first one has A on the left side alone, and B + 350,000 on the right, yes? Because A is alone on the left side, that means it's already isolated. That means you can put the right side of the first equation (B + 350,000) in place of A in the second equation, because that equals A.
So, let's take the second equation and substitute A for \[B + 350,000\]So the only variable you have is B.