To solve each of these proportions:

1) 8/9 = 3/4x/2

First cross multiply to get:
8 2 = 9 3/4x

16 = 27/4x

Multiply both sides by 4 to get rid of the fraction:
4 * 16 = 27x

64 = 27x

Divide by 27 to solve for x:
x = 64/27

2) 8 + 5/76 = 5/9x

First convert the fraction into a decimal for easier calculations:
5/76 = 0.0658

So the equation becomes:
8 + 0.0658 = 5/9x

8.0658 = 5/9x

Multiply both sides by 9 to get rid of the fraction:
9 * 8.0658 = 5x

72.5922 = 5x

Divide by 5 to solve for x:
x = 72.5922 / 5

3) 12/7 = 9/x

Cross multiply to get:
12x = 7 * 9

12x = 63

Divide by 12 to solve for x:
x = 63 / 12