Find the area of a square whose perimeter is 3 times the perimeter of a rectangle with sides of 3 cm and 5 cm.

1. Find the perimeter of the rectangle using the formula:

P = (a + b) * 2, where a and b are the lengths of the sides of the rectangle.

(3 + 5) * 2 = 24 (cm) perimeter of the rectangle.

2. Find the perimeter of the square to do this, increase the perimeter of the rectangle by three times.

24 * 3 = 72 (cm) perimeter of the square.

3. Find the side of the square by dividing its perimeter by the number of sides.

72/4 = 18 (cm) side length of the square.

4. Find the area of a square by squaring the length of its side.

18 2 = 324 (cm2) area of this square.

Answer: 324 cm2.