the complete question in the attached figure
Let
x------> length side of a cube
we have that
[surface area original]=18 units²
surface area original=x²*[number of faces]
number of faces=2*[4+4+1]-----> 18 faces
surface area original=x²*[18]
18=18*x²-------> x²=18/18---------> x=1 unit
If a cube is added on the green face,the new surface area will be
18+4*[area of one faces]-----> 18+4*1²----> 22 units²
the answer is
22 units²
