You find its area.
Multiply the length and width to find the area of both a rectangle and square.
multiply the length and the width
Multiply the length by the width.
multiply length times width
Multiply its length by its width.
multiply length and width
length times width
multiply length times height times width
multiply them
area
multiply width time area
You cannot find the area of a rectangle if you only know its length. You need to know its width too. Then you multiply its length by its width.