jmc

We can line up the numbers and subtract just as we do in base 10. For example, when we borrow from the $3^1$s place, the digit 1 in the units place becomes $4$, while the digit in the $3^1$s place decreases by 1. Continuing in this way, we find \begin{array}{c@{}c@{}c@{}c@{}c} & \cancelto{1}{2} & \cancelto{4}{1} & \cancelto{1}{2} & \cancelto{4}{1}_3 \\ -& & 2 & 1 & 2_3 \\ \cline{2-5} & 1 & 2 & 0 & 2_3 \\ \end{array}So the difference is $\boxed{1202_3}$.