Find the side length of a square whose perimeter is equal to the perimeter of a rectangle with sides of 7 cm and 5 cm.

We find the perimeter P of a rectangle with sides equal to 7 cm and 5 cm as twice the sum of the lengths of these sides:
P = 2 * (7 + 5) = 2 * 12 = 24 cm.
Find the side length of the square.
Let us denote it by x.
According to the condition of the problem, the perimeter of this square is equal to the perimeter of a rectangle with sides equal to 7 cm and 5 cm, therefore, we can draw up the following equation:
4 * x = 24.
We solve the resulting equation and find the length of the side of the square:
x = 24/4;
x = 6 cm.
Answer: the length of the side of the square is 6 cm.