what would happen if you added the original equations instead of subtracting?

Answer 1

In what context?

If you are speaking about the elimination method in linear equation systems, I would caution you that elimination doesn't always involve subtraction...it involves eliminating one variable and sometimes that involves subtraction but sometimes addition. Example:

x+y=5

x-y=1

To solve you add to eliminate the y.

And in this set you could also subtract to eliminate the x.