-X1 +3X3-X4=8
5X2+11X3+9X4=23
3X2+5X3+7X4=9

X1=3X3-X4-8
X4=3X3-X4-8
X3=X1/3+X4/3=8

3X2+5X3+7(3X3-X4-8)=9
X2=-26X3/3-7X4/3+65/3 smile.gif