120 cm²
1) Let's calculate the surface area by calculating each polygon's area. In this polygon we have, considering that this is a building block
2 triangles 4x3
2 triangles = 12 cm²
1 rectangle 6 x 9:
1 rectangle 5 x 9
1 rectangle 3 x 9
2) Adding it all up: 12 cm²+54+45+27 = 120 cm²