1•a-3•b+4•c=100 (1)

4•a-6•b+9•c=999 (2)

2•a-6•b+7•c=879 (3)

(1)x3, get

2•a-6•b+8•c=200 (4)

(4)-(3), get

c=-679

(2)-(3), get

2•a+2•c=120

a=[120-2•(-679)]/2=739

substitute value a & c to (1), get

-3•b=100-1•(739)-4•(-679)=2077

3•a-3•b+6•c

=3•(739)+2077+6•(-679)

=220

3-3+6=220