a-3=0
b+1=0
∴a=3
b=-1
(2b-a+m)/2的值比1/2b-a+m多1
(2b-a+m)/2=b/2-a+m+1
(-2-3+m)/2=-1/2-3+m+1
-5+m=-1-6+2m+2
m=0