To find what "a" is equal to, you need to get it alone. There are no common variables here or any numbers, so you just need to move the "k" to the other side. So, since it is written out to be "a" multiplied by "k" the opposite of multiplication is division. ak/k = a. And anything you do to one side, you have to do to the other side. (v+w) divided by k. Which there are no numbers so you get (v+w)/k.
