To some, "Now we can add the two equations together to eliminate y: might need a little explanation. The (an) answer is that since the LHS and RHS are equal, y

To some, "Now we can add the two equations together to eliminate y: might need a little explanation. The (an) answer is that since the LHS and RHS are equal, you can choose to add or subtract them to another equation and preserve equality. If I remember correctly, substitution (isolating x or y) was introduced before this technique.