The perimeter of an equilateral triangle is 27 cm, and the square is 3 cm less than the area of this square.

1. If the perimeter of the square is 3 cm less than the perimeter of the triangle, we find the perimeter of the square by subtracting 3 cm from the perimeter of the triangle:
27 cm – 3 cm = 24 cm.
2. Determine what the side of the square is equal to by dividing the perimeter into 4 parts:
24 cm / 4 = 6 cm.
3. Determine what the area of the square is equal to by the formula S = a ^ 2, where S is the area of the square, and is the side of the square:
Area = (6 cm) ^ 2 = 36 cm2.
Answer: the square has an area equal to 36 cm2.