Answer
The side length of the square is
$10r-11$.
Work Step by Step
$100r^{2}-220r+121=$
...write first and last terms as squares.
$=(10r)^{2}-220r+(11)^{2}$
...does middle term equal $2ab$?
$220r=2\cdot(10r)\cdot(11)$
$=(10r)^{2}-2\cdot(10r)\cdot(11)+(11)^{2}$
...write as the square of a binomial.
$=(10r-11)^{2}$
The side length of the square is
$10r-11$.
