Answer:
10.5:12:13.5
Explanation:
The only two values you know and can compare are 8 and 12, so you have to find the relationship between them. 12/8 = 1.5. To get the two unknown values, you multiply 1.5 by the ones you do know.
7 * 1.5 = 10.5
9 * 1.5 = 13.5
So, you get 10.5:12:13.5