add to both sides 



The  and the  cancel out on the left
The  and the  combine to give 6 on the right:





add to both sides



The terms on the left side combine to give 
The  and the 
cancel out on the right, and you have this left:



Now we divide both sides by the coefficient of n which is 3



The 3's on the left cancel and leaves n.



The 6 divides by 3 on the right and gives 2



Edwin