5x+2y=39,(k-2)x+(k+4)y=39
得: 5x+2y=(k-2)x+(k+4)y
y=(7-k)x/(k+2)
由x>3y>0,有(7-k)/(k+2)>0
解得：-20,有
x>3(7-k)x/(k+2)>0
则x-(21-3k)x/(k+2)>0
有(kx+2x-21x+3kx)/(k+2)>0
(4k-19)x/(k+2)>0
解得：k>4.75, 或 k