X+ y = 3k x - y = k the solution of the system shown is

First, you have  x+y= 3k, x-y= k     and what you need to do now is eliminate the y's because they have the same sign (- and +). So now you have x= 3k, x=k. You so then you add the x with the other x (2x) and the 3k with the k (4k). Now you have 2x = 4k , which is the same as x = 2k. Now that you now x in terms of k, you can put it in the very first system, like this: 2k+y=3k, 2k-y=k. If you solve them, first one and then the other, you will get the same result: y=k.

So, you can say that x=2k and k=y. 
Hope that was helpful, have a good day! :))