The area of a rectangle is 256.5 m . if the length is 18m what is the perimeter of the rectangle

First we have to find the width. The area of a rectangle is width times length, so the width * 18m = 256.5m. Divide both sides by 18m to isolate the width, and you get width = 256.5m/18m. Therefore, width = 14.25m.
To find the perimeter of the rectangle, you add up all 4 sides. Two of these sides are the width, and the other two are length, so you multiple both by two and add them together. 18*2 + 14.25*2 = 36 + 28.5 = 64.5, so the perimeter is 64.5m.