Solve by the addition-or-subtraction method.

1. 2x + y = 18

2x - y = 22

2. 12a - 5b = 18

15a - 5b = 30

Solve by either the substitution or the addition-or-subtraction method.

3. 2m = n - 3

3m = 2n - 9

Can someone please explain to me what these methods are and how to use them? And what is the differnece between these?...Thanks for the help!