To find the area of a square, we multiply the length by its width
In this case, the length and width of the square area are both 9!
We know this becasue it says "each side of a square piece of cardboard is 9 inches long"
Now let's solve the problem
Area = L × W
Area = 9inches × 9inches
Area = 81 inches²
*I put the "²" above inches because any area of a square is just multiplying the same number by itself or ... in other words... "squaring it"
[ex: 9² = 9 × 9]