$x = 9$
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.
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