Evaluate 21-9

Set up the subtraction problem in long subtraction format.

Borrow ${\displaystyle 10}$ from the digit in the next column to the left. Add the borrowed ${\displaystyle 10}$ to ${\displaystyle 1}$. Reduce the number that was borrowed from by ${\displaystyle 1}$.

Subtract ${\displaystyle 9}$ from ${\displaystyle 11}$.

Subtract ${\displaystyle 0}$ from ${\displaystyle 1}$.