I guess this depends on how much of an answer your instructor is expecting.  Your answer is definitely correct.  But I do not know if it covers all the points your instructor wants.

You do a very good job of describing what realists think -- the fact that they say states act only based on what will give them more power or security.

What you do not do is to talk about why the USSR thought this action was in their national interest.  I would talk some about how Russia had been invaded at least twice by states from Western Europe.  Because of this, the Soviets would have believed that this region could pose a threat to them.  In addition, they believed that the US could pose a threat and that the US threat would come from that direction as well.  Because of that, they thought a buffer zone was in their national interest.