Answer:
9 + x
Step-by-step explanation:
The area of a rectangle is length times width.
A = lw
We know the area is 81 - x^2 and the width is 9 - x, so plug them in.
81 - x^2 = 9-x * w
81 - x^2 is a difference of squares in the form a^2 - b^2 = (a-b)(a+b), so we know the width must be 9 + x.
Test it to be sure:
(9 + x)(9 - x)
81 + 9x - 9x -x^2 = 81 - x^2
It works!
