The diagonal divides a pentagon with a perimeter of 45cm into a quadrangle with a perimeter of 30cm

The diagonal divides a pentagon with a perimeter of 45cm into a quadrangle with a perimeter of 30cm and a triangle with a perimeter of 21cm. How long is this diagonal?

Let the diagonal length be d. If we subtract the length of the diagonal from the perimeter of the quadrilateral and the perimeter of the triangle, then we get the perimeter of the pentagon:
(30 – x) + (21 – x) = 45 cm;
51 – 45 = 2x;
x = 6/2 = 3 cm.
Answer. The diagonal length of the pentagon is 3 cm.