4x+9y=12 (1)
3y-2z=1 (2)
7x+5z=4又3/4
28x+20z=19 (3)
(1)-(2)*3
4x+6z=9 (4)
(4)*7-(3)
42z-20z=63-19
22z=44
所以
z=2
x=(9-6z)/4=-3/4
y=(2z+1)/3=5/3