rref ((-3,0,7,0),(-9,0,21,0),(0,0,2,8),(0,0,1,4),(3,0,13,4),(0,0,1,0)

Uh oh! Wolfram|Alpha doesn't run without JavaScript.

Please enable JavaScript. If you don't know how, you can find instructionshere.Once you've done that, refresh this page to start using Wolfram|Alpha.