Answer
$123_{four}$
Work Step by Step
$(3\times 1)_{10}=(3)_{10}=(3)_{4}$
Record the $3$; there is nothing to carry.
$\begin{array}{llll}
& 2 & 1_{four} & \\
\times & & 3_{four} & \\
-- & -- & -- & \\
& & 3_{four} &
\end{array}$
$(3\times 2)_{10}=(6)_{10}=1\times 4+2=(12)_{4}$
Record the $12.$
Result: $123_{four}$
