21. \( (2.1)^{2}= \)

To solve \( (2.1)^{2} \), simply multiply 2.1 by itself: \( 2.1 \times 2.1 = 4.41 \). So, the answer is \( 4.41 \). Squaring numbers is a fundamental concept in mathematics, often used in geometry when calculating areas. For instance, if you have a square with a side length of 2.1 units, the area will be the square of that length, which gives you that neat \( 4.41 \) square units of space to work with!