If you want to check if (1,2) is a solution to the system, you have to plug the x and y values back into both equations. If they work for one equation, but not the other, than the coordinates are not a solution to the system.
3(1) - 4(2) = -5
3 - 8 = -5
-5 = -5
2 = 4(1) - 2
2 = 4 - 2
2 = 2
Since both of these checks are true, then (1,2) is a solution to the system.