The area of a circle is A = πr². The areas are 9π cm² and 25π cm², so the ratio of their areas is 9:25.

By the triangle inequality theorem, the length of the third side must be greater than the difference of the other two sides and less than their sum: |7 - 10| < x < 7 + 10, which gives 3 < x < 17.

The maximum length of the third side is less than the sum of the other two sides: 8 + 15 = 23 cm, so the maximum possible length is 22 cm.

The circumference of a circle is given by C = πd. For a diameter of 12 units, C = π * 12 = 12π units.