A number of different things could go in place of the box to make it no solution. One example would be 3x - 1.
You can tell this by looking at the equation and distributing the 3 and simplifying.
3(x + 1) + 9
3x + 3 + 9
3x + 12
Now, to write something that gives us no solution on the other side, we keep the same value for the coefficient of x, but a different number for the constant. If we use the example from above, you'll get an improper answer.
3x + 12 = 3x - 1 ----> subtract 3x from both sides
12 = -1 (Since this is an untrue statement, we know that this has no solution).