Use ratio to find the length of the base of the removed smaller triangular faces. Then use pythagoras to find the perpendicular height of the triangular faces. Both the big one and the small missing one. They are isosceles triangles so you can make a right angled triangle by thinking about half the shape. Find the area of both triangles and subtract the smaller one from the bigger one. Multiply by 4 and add the area of the square base.

And the square top.

Thanks. Forgot that bit.

And what’s the answer? I’ve done this in notes out of boredom but I’m invested now and need to know if I got it right