Thinking Mathematically (6th Edition)

$3_{four}$
$1$ is less than $2$, borrow one four from the next column $(4+1)-2 =3_{ten}=3_{four}$ record the 3. $\left[\begin{array}{lll} & 2^{(*)} & 1_{four}\\ - & 1 & 2_{four}\\ - & - & -\\ & & 3_{four} \end{array}\right]$ Next column: we borrowed 1 from 2, subtract $1-1=0_{ten}=0_{four}$ No need to record the 0, Result = $3_{four}$ Check in base 10: $9-6=3$