Cube

Original surface area    21 x 21 x 6 sides =2646 cm^2

New surface area    21 x 21 x 3 sides    +  15x15x3   +  (21 x 21 - (15x15) ) x 3

                                   1323                +          675            +     648

                                     2646 cm^2

No change in surface area

Thanks, EP.....to see this more clearly.....note that  the cube that is removed had three exposed sides contributing to the original surface area of the original cube

But...note when the smaller cube is removed....the same surface areas remain on the  altered cube