"Solve for" means isolate, or get by itself. To isolate variables, perform inverse operations:
a.
3(2x - 1) + 2y = 5x
6x - 3 + 2y = 5x
x - 3 + 2y = 0
x - 3 = -2y
3 - x/2 = y
y = 3 - x/2
There - you've isolated y.
b.
600x + 200y = 500x
100x + 200y = 0
100x = -200y
x = -2y
And that's your isolated term for x.
Hope this helps. (:
