The sum of the lengths of two different sides of a rectangle is 7 cm. Find the perimeter of this rectangle.

The perimeter of a rectangle is the sum of the lengths of all its sides. Since in a rectangle the opposite sides are equal, then P = 2 * (a + b), where a is the length, b is the width. Proceeding from the fact that in the condition the sum of different sides is given, then these sides are length and width. Knowing that the sum of the length and width is 7 cm, we calculate the perimeter.
P = 2 * (a + b) = 2 * 7 = 14 cm.
Answer: 14 cm.