plug the first equation into the second
-x + (x + 1) = 3
1 = 3
This is obviously false, so that means the two lines are parallel and have no solution
If we rarrange -x + y = 3, we get
y = x + 3
And the first equation was y = x + 1
You can see that both have the same slope but different y-intercepts, which means that the two lines are parallel and have no solution.