The perimeter of a rectangle is 42 cm. Calculate its area if you know that one of its sides is 9 cm.

Let us express the length of the rectangle from the formula for determining the perimeter of such a shape.

If: P = 2a + 2b, therefore: a = (P – 2b): 2.

According to the condition of the assignment, the sum of all sides of the rectangle = 42 cm, and the width = 9 cm:

a = (42 – 2 * 9): 2 = 24: 2 = 12 cm.

The area of the rectangle, since its opposite sides are equal, is calculated by the formula:

S = a * b = 12 * 9 = 108 cm².