3/9=b/12
b/5=6/15
2/9=8/b
10/b=30/33

3/9=b/12

9b = 12*3

9b = 36

b = 36 : 9

b = 4

---------

b/5=6/15

15b = 6*5

15b = 30

b = 30 : 15

b = 2

--------------

2/9=8/b

2b = 9*8

2b = 72

b = 72 : 2

b  36

-----------

10/b=30/33

30b = 33*10

30b = 330

b = 330 : 30

b = 11