Jason, the reason 3x and 4x² cannot be subtracted (or for that matter, added) is because they are two completely different terms. The difference in the variables shows that they are different terms. So the terms would either have to be 3x and 4x or 3x² and 4x² for one to add or subtract them.
On a side note: you post many calculus problems yet you do not know that 3x and 4x² cannot be subtracted. How is that so?