I need o fine the length and the width of the rectangle that haves the area 9in and the perimeter

The perimeter = (2 lengths) + (2 widths) = 12

(1 length) + (1 width) = 6

Area = (length) x (width)

So you need two numbers that add to 6 and multiply to 9.

How about 3 and 3?

Area = 3 x 3 = 9

Perimeter = 3 + 3 + 3 + 3 = 12

It works, and your rectangle is actually a square!