Can you show your work?
Here's how you can do an extended matrix in LaTeX (double-click to see the source code):
{ -.5x + .5y + .5z = 1.5
4.2x + 2.1y + 2.1z = 0
.2x + ___ + 0.2z = 0
The answer is supposed to be (-1, 1, 1) but i got (10/21, 0, 10/21). I checked my answer by substituting my answer for the variables in the 1st equation and got 3 instead of 1.5.