Answer:
9 + x
Step-by-step explanation:
81 - x² ← is a difference of squares which factors in general as
a² - b² = (a - b)(a + b), thus
81 - x²
= 9² - x² = (9 - x)(9 + x)
Area = length × width = 81 - x² = (9 - x)(9 + x)
Since width = 9 - x then length = 9 + x
