Hello, r_maths!

You'rethat closeto the correct answer . . .

The surface area of the original cube is: .6 × 7² = 294 m².

When that corner is sliced off, welosethree isosceles right triangles

. . Each has 4m legs and an area of 8 m².

Then wegainan equilateral triangle with side 4√2 m.

. . Its area is: 8√3 m².

The final surface area is: .294 - 3(8) + 8√3 .≈ .283.86