Explanation:
I guess you mean it is
12 cm long, 4cm wide and 5 cm high.
and the fabric goes outside on the box ? strange, but ok.
a box (like a die) has 6 sides in the form of 3 pairs of equal rectangles :
top-bottom, left-right, front-back
the surface area is the sum of all 6 rectangles.
top-bottom :
2 × 12×4 = 2 × 48 = 96 cm²
left-right :
2 × 4×5 = 2 × 20 = 40 cm²
front-back :
2 × 12×5 = 2 × 60 = 120 cm²
in total
120 + 40 + 96 = 256 cm²