Use common denominators to determine whether the ratios form a proportion.
6 : 9 and 9 : 12
a.
yes. they form a proportion.
b.
no. they do not form a proportion.

They are not in proportion, since its 6:9 round it down (2:3) so pretend the 6 are apples and the 9 are oranges

Every 2 apples there are 3 oranges

So having 9 apples what is that?

2 x ? = 9

2 x 4.5 = 9

So if you want to keep 2:3 you have you times the oranges by 4.5 aswell!

3 x 4.5 = 13.5

So since it is 9:12 not 9:13.5 they are not in proportion