-6<=4m^2-9m
4m^2-9m+6>=0
4(m-9/8)^2+15/16>=0
肯定成立
4m^2-9m<= -2
4m^2-9m+2<=0
(4m-1)(m-2)<=0
1/4<=m<=2
所以1/4<=m<=2