Answer
$111_{three}$
Work Step by Step
Start with right-most column.
0 is less than 2, we need to borrow 1 three...
we can't borrow from zero, so we borrow 1 from $20_{three}$.
This leaves 12 instead of 20 in $1200_{three}$in the top row...
After borrowing:
In base 10, $(0+3)-2=1$, which we record.
Also, adjust the top row to reflect the borrowing.
$\begin{array}{lllll}
& 1 & *1 & *2 & 0_{three}\\
- & 1 & 0 & 1 & 2_{three}\\
-- & -- & -- & -- & --\\
& & & & 1_{three}
\end{array}$
Next column: $ 2-1=1. $
Record the 1.
$\begin{array}{lllll}
& 1 & *1 & *2 & 0_{three}\\
- & 1 & 0 & 1 & 2_{three}\\
-- & -- & -- & -- & --\\
& & & 1 & 1_{three}
\end{array}$
Next column: $1-0=1,$
Record the 1.
$\begin{array}{lllll}
& 1 & *1 & *2 & 0_{three}\\
- & 1 & 0 & 1 & 2_{three}\\
-- & -- & -- & -- & --\\
& & 1 & 1 & 1_{three}
\end{array}$
Next column:$ 1-1=0$
Result: $111_{three}$
