x的值比y的值小5
y-x=5
x-y=-5
x-y=9m=-5
m=-5/9
x+2y=3m (1)
x-y=9m (2)
(1)*5+(2)*4
5(x+2y)+4(x-y)=5*3m+4*9m
9x+6y=51m
3x+2y=17m=17
m=1