x + y = - 0.3333333 That's the decimal equivalent of - 1/3
x - y = 18 Add these two together.
2x = 17.66666 Divide by 2
x = 8.833333
8.83333 + y = - 0.333333
y = - 0.333333 - 8.83333
y = - 9.16666
You should check this using the difference.
It will not come out exactly, but you do get a bunch of nines.