How do you solve proportions with ratios with a letter

SORRY if its too late. 

Proportions with variables: 

let's look at an example from my homework hehe 

.3:4=9:y 
First, you change the ratios to fractions. 
.3/4=9/y. 
4 times 9 is 36. I multiplied the numerator of 9/y and the denominator of .3/4. 
You also do this with the other numerators and denominators. .3 times y= 36. (Both equations have to equal the same number) if you don't know the answer, just do 36 divided by .3. The answer is 120. 
y=120