You may isolate either variable, it doesn't matter. In this case, though, don't you think elimination would be better?
We can multiply the first equation through by 2, and get a new set of equations:
2x + 10y = -26 -- 1
2x - y = 7 -- 2
Subtracting 2 from 1, we have:
10y -(-y) = -26 - 7
11y...