For this case we have that by definition, the area of a rectangle is given by:
[tex]A = w * l[/tex]
Where:
w: Is the width of the rectangle
l: is the length of the rectangle
According to the data of the statement we have:
[tex]A = 81-x ^ 2\\w = 9-x[/tex]
So, the length is given by:
[tex]l = \frac {A} {w}\\l = \frac {81-x ^ 2} {9-x}[/tex]
We factor the numerator:
[tex]l = \frac {(9 + x) (9-x)} {9-x}[/tex]
We simplify:
[tex]l = 9 + x[/tex]
So, the length is [tex]9 + x[/tex]
Answer:
[tex]9 + x[/tex]
