SOLUTION: If x:y = 3:5 and x:z = 5:7, what is the value of y:z ? a. 3:7 b. 21:25 c. 7:3 d. 25:21

Instead of me just doing it for you, I'll do one EXACTLY like it.
Then you can use it as a model to do yours.

x corresponds to 4 in the first proportion
x corresponds to 3 in the second proportion.

Get the LCM of 4 and 3 which is 12.

So change the ratio 4:9 to the ratio 12:27 by multiplying 4 and 9 by 3
Change the ratio 3:8 to the ratio 12:32 by multiplying 3 and 8 by 4

 x:y = 12:27 and x:z = 12:32

Now that x corresponds to 12 in both proportions, y corresponds to 27
and z corresponds to 32, so

 y:z = 27:32

Now use that as a model and do yours exactly the same way.

Edwin