Fists we are going to sketch our cardboard sheet; since the length and the width are equal, we know that our figure will be a square in which each side will be 6 foot long. Now, we need to cut an equal size square form each corner, but we don't now how long the lengths of the square will be, so we are going to assign the variable [tex]x[/tex] to each sides of our square as you can see in the picture.
