2元1次方程x+2y=9<1>3x
x+2y=9 <1> 3x-2y=-1 <2>
x+2y=9 (1) 3x-2y=-1 (2) (1) x=9-2y 3x-2y=-1 3(9-2y)-2y=-1 -8y=-28 y=7/2 ===>x=9-2y=9-7=2