Write the sentence as a proportion
5 is to 7 as 15 is to x
9 is to 3 as x is to 12
g is to 9 as 16 is to 12
6 is to 18 as y is to 3

"is to" can be another word for ":" or a proportion.

5:7 as 15:21 (x = 21)

9:3 as 36:12 (x = 36)

12:9 as 16:12 (g = 12)

6:18 as 1:3 (y = 1)