Answer:
9+x
Explanation:
The area of a rectangle is the width multiplied by the length. Working backwards, then, you can find the width or the length by dividing the area by the other one. In this case, the length of the rectangle is the area, or 81-x^2, divided by the width, 9-x. 81-x^2 factored out is (9-x)(9+x), so dividing this by 9-x yields a length of 9+x. Hope this helps!