Algebra: A Combined Approach (4th Edition)

$x = 9$
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.