Question 862301
4(x-120)=3x 
4x-480-3x
x-480=0
x=480
x is the speed on the outbound trip 
x-120=360 return trip


3(y+120) = 4y 
3y+360=4y
360=y the return
y+120=480 the  outbound trip there
y is average speed on the return trip

Which should you use?  

If you are only interested in the outbound then use the first 4(x-120)=3x 
If you are only interested in the return then use the second 3(y+120) = 4y 
If you want to find out both speeds then it doesn't matter which one you use.