m-n+4的绝对值≥0,
(n-3)^2≥0,
m减n加4的绝对值与(n减3)的平方互为相反数,则
m-n+4=0,m-n= -4
n-3=0,n=3
所以,m= -1
m^2-n^2= (-1)^2- 3^2= -8