3x + y =15,-x + 2y = -19
3x + y = 15 ---> 3x + y = 15
-x + 2y = -19 ---> x 3 ---> -3x + 6y = -57
Add down the columns: 7y = -42
Divide by 7: y = -6
Replace into the first equation: 3x + y = 15 ---> 3x + (-6) = 15 ---> 3x = 21 ---> x = 7