Find the area of a rectangle whose side is 7 cm and the perimeter is 26 cm.

Perimeter is the sum of all sides, it is calculated using the formula:
P = 2 * (a + b), where a and b are the sides of the rectangle.
The area of the rectangle is calculated by the formula:
S = a * b;
Substitute these values into the perimeter formula, and calculate the length of the unknown side.
Let a = 7 cm, then:
26 = 2 * (7 + b)
26 = 14 + 2 * b;
2 * b = 26 -14;
2 * b = 12;
b = 12: 2;
b = 6 cm;
Find the area:
S = 6 * 7 = 42 cm ^ 2.