{6x-2y=2118+2y+6x=30y
{6x-2y=21
18+2y+6x=30y
6x-2y=21(1)
所以6x-28y=-18(2)
(1)-(2)
6x-2y-6x+28y=21+18
26y=39
y=3/2
x=(21+2y)/6=4