We must first determine the total perimeter of the rectangle, then divide this amount by 4 (because a square has four equal sides) to find the sides of the square.
The perimeter of a rectangle is $\(2(l+w)=2(10 +8)=36\)$.

Therefore, each side of the square is $\(\frac{36}{4}=9\)$.
Now that we have the sides of the square, we can find the area by squaring the length of one side.
Area of square $\(=9^2=81\)$.
The quantities are equal; the answer is (C).