A plane figure with four straight sides and four right angles, especially one with equal adjacent sides, in contrast to a square with hollow at some thickness.
Length (L) = 10 cm , Thickness (T) = 3
Outer Square = Length(L) * Length(L)
Hollow Square = (Length(L)-( T * 2 )) * Length(L)-( T * 2 )
Area (A) = Outer Square - Hollow Square
Result = 100 - 16 = 84 m2