The height of the rectangular parallelepiped is 18 cm. It is equal to the difference between the length and width

The height of the rectangular parallelepiped is 18 cm. It is equal to the difference between the length and width of the parallelepiped, and the length is 4 times the width. Find the area of the side faces.

We define that the width of the parallelepiped is x cm, then the length is 4x cm. By the condition, it is known that the difference between these values is 18 cm. Let’s compose and solve the equation:
4x – x = 18
3x = 18
x = 6 (cm) – width;
4x = 4 * 6 = 24 (cm) – length.
All parameters are known, we find the area of the side faces by the formula:
S = 2 * (a * b + b * h + a * h) = 2 * (24 * 6 + 6 * 18 + 24 * 18) = 2 * (144 + 108 + 432) = 2 * 684 = 1368 ( cm²).
Answer: the area of the side faces is 1368 cm².