x+y=m (1)
5x+3y=31 (2)
(2)*5-(2)
5x+5y-5x-3y=5m-31
2y=5m-31
y=(5m-31)/2
x=m-y=m-(5m-31)/2
非负数则x>=0,y>=0
x=m-(5m-31)/2>=0,2m>=5m-31,m=0,m>=31/5
31/5