Answer
$(m+9)^{2}$
Work Step by Step
$m^{2}+18m+81=$
...write the last term as a square.
$=m^{2}+18m+9^{2}$
...does middle term equal $2ab$?
$18m=2(m)(9)$
$=m^{2}+2(m)(9)+9^{2}$
...write as the square of a binomial.
$=(m+9)^{2}$
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