Problem
Faces meeting at a corner all touch, so A touches E and F in one view and C and D in another.
Three faces sharing a corner are like three walls of a room meeting at a point — each pair clearly touches, so each is perpendicular to A.
6.G.A.4Visualize Spatial RelationshipsCross off A and its four neighbours C, D, E and F, and the only letter left is B.
A cube has 6 faces; one is A, four are perpendicular to A, so exactly one is left — and that one has to be the opposite face.
5.MD.C.3Make A Systematic ListA cube face meets four others and faces exactly one, so the leftover letter is A's opposite.
If you build the cube and pair up the faces, every face gets exactly one partner across from it, and A's partner comes out to be B every way you check.
6.G.A.4Create A Physical RepresentationA face of a cube is perpendicular to four faces and parallel to exactly one.
The six faces fall into three pairs that face away from each other across the cube, so each face has exactly one partner it never meets.
The four faces left over each share an edge with it, and those four together with the face itself and its partner account for the whole surface.
On a cube, list every face that touches the one you care about — the only letter left over is the one stuck on the opposite side.