The answer depends on what information you have about the rectangle: the area and width, or width and diagonal, area and perimeter or some other measures.
Volume=length x width x height (V=lwh)
area of the rectangle..
A = lw Area of a rectangle = length times width
length * width
Length times width.
Area of rectangle is l x b. Where l is length and b is breath of the rectangle.
With X length = square
Length x Width LxW
length times width
Knowing the length and the width.
in finding the area if a square or rectangle multiply the length and width, for a triangle, multiply length and width divided by two.
If you do not know the length or breadth of a rectangle, you cannot know the area. If you do not know the area of a rectangle, you cannot know the length and breadth. To know the length and breadth of a rectangle, you have to know some other contributing factor in the equation. If you don't, measure it!