2x-y=3m
x+2y=4m+5
x+y=9
1式乘以a+2式乘以b凑成3式,因此
2a+b=1
-a+2b=1
3m*a+(4m+5)*b=9
a=1/5 b=3/5
(3/5)m+3*(4m+5)/5=9
3m+3=9
m=2