So the augmented matrix is:
2 0 4 -2
-1 2 -3 4
1 2 1 k
I assume you know how to row reduce? The k can be treated just as any constant....i.e. "do things to it"
First things first, divide the first row by 2. Then start adding multiples of the first row to the second (since the first entry of row 2 is -1, the choice should be obvious). Continue through to row 3 after you have entry (2,1) as 0. Then after that, work backwards. Don't let k trick you, treat it like a number but remember you can put things like "k+2" if you need.