Hello algebraisabeastAs the pyramid is cut off it reveals a new triangular surface whose area must now be included in the total surface area of the new solid.

If the cut is made through three of the cube's vertices (which I assume to be the case), then, as you say, the area removed is . But the new area revealed is an equilateral triangle whose sides are units long, and whose area (using area of triangle = ) is

So the new area is (to 2 d.p.)

Grandad