1.x+2y=-14 3x-y=0
y=3x带入x+2y=-14 得x+6x=-14 ,则x=-2,y=-6
2.2x-y=1 3x-2y=5
2x-y=1得y=2x-1代入3x-2y=5,得3x-4x+2=5,则x=-3,y=-7
3.x+y=3 2x+4y=5
因为x+y=3,则2x+4y=2x+2y+2y=2(x+y)+2y=6+2y=5,得y=-1/2,x=7/2
4.9a-13b=-12 a+3b=2
a+3b=2,则a=2-3b代入9a-13b=-12 ,得18-27b-13b=-12,则b=3/4,a=-1/4