Eduardo is building a sandbox that has an area of 84 square feet. what are the possible whole number measurements for the length and width of the sandbox?

To find the area, of a rectangle, you multiply length by width. Therefore, any factor pairs of 84 would be possible measurements for the length and width of the sandbox.
length=1ft, width=84ft (1x84=84)
length=2ft; width=42ft (2x42=84)
length=3ft; width=28ft (3x28=84)
length=4ft; width=21ft (4x21=84)
length=6ft; width=14ft (6x14=84)
length=7ft; width=12ft (7x12=84)
And switching the numbers around gives you more possibilities.
length=12ft; width=7ft
length=14ft; width=6ft
length=21ft; width=4ft
length=28ft; width=3ft
length=42ft; width=2ft
length=84ft; width=1ft
12 total possible whole number pairs for the length and width of the sandbox.
See how that works? Easy as pie, so long as you have the patience to find all the factors.
I hope this helps!