先解2x-3y=3 (1)
x+4y=7 (2)
(2)×2-(1)
8y+3y=14-3
y=1
x=7-4y=3
代入另两个
3a+b=-1 (3)
6a+3b=3
即2a+b=1 (4)
(3)+(4)
5a=0
a=0,b=-1
a²-b²=0-1=-1