do the variable have to be in a certain group

Question

do the variables have to be in a certain group

Answer 1

As long as the variables are the same ussually you can combine them. For examble if you have the equation: 2x+(3x-6y)=9y+x

you can combine the the x's on the right side even though they're not grouped together.

2x+3x=5x so you have 5x-6y=9y+x and you can even combine by subtracting x from both sides getting 

4x-6y=9y then add 6y to both sides to get 4x=3y. I hope this example was able to clear up some questions you may have had.