case 1:B=Φ
7-m < m-4
2m>11
m>11/2
case 2:B≠Φ ie m≤11/2
B⊆A
=>
-2≤m-4 and 5≥7-m
m≥2 and m≥-2
m≥2
solution for case 2 :2≤m≤11/2
B⊆A
case 1 or case 2
ie m≥2