Close rectangular box has three couples of identical parallel faces.
there are two faces 6''×5'',
two faces 6''×3''
and two faces 5''×3''
So, the area of the box would be the total of the areas of the faces:
A = 2· 6·5 + 2· 6·3 + 2· 5·3 = 2·30 + 2·18 + 2·15 = 60 + 36 + 30 = 126 [square inches]