The lengths of the three sides of a triangle are in the ratio 3:5:6. The perimeter of the triangle is 21 cm. Find the length of each side of the triangle.
a) 3 cm, 5 cm, 6 cm
b) 6 cm, 10 cm, 12 cm
c) 9 cm, 15 cm, 18 cm
d) 12 cm, 20 cm, 24 cm