Let's solve these proportions:

1) 8/9 = 3/4
Cross multiply:
8 4 = 32
9 3 = 27

So, 8/9 = 3/4 simplifies to 32 = 27, which is not true. Therefore, there is no solution for this proportion.

2) x/2.8 = 5/7
Cross multiply:
x 7 = 2.8 5
7x = 14
x = 2

So, x = 2 is the solution for this proportion.

3) 6/5 = 9/x
Cross multiply:
6 x = 5 9
6x = 45
x = 45/6
x = 7.5

So, x = 7.5 is the solution for this proportion.