#### Answer

$x = 9$

#### Work Step by Step

The are of the square is $x^2$
let's compare it to 81.
$x^2 = 81$
$x^2 - 81 = 81- 81 = 0 $
$(x-9)(x+9)=0$
$x = 9$
And
$x\ne -9$
because it is a length, and a length cannot be negative number.

Published by
Pearson

ISBN 10:
0321726391

ISBN 13:
978-0-32172-639-1

$x = 9$

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