a) The internal surface area will just be the area of the original rectangle less the area of the 4 squares cut away
So we have
(1200)(1000) - 4 ( 300)^2 =
1200000 - 4 ( 90000) =
120000 - 360000 =
840,000 mm^2
b ) When the squares are cut out, the height of the box will be 300mm
The width will be (1200 - 2(300) ) = ( 600)mm
The length will be (1000 - 2(300)) - (400)mm
So the volume will be
(300 * 600 * 400) = 72,000,000 mm^3