Explanation:
we have 2 blocks, as the graphic shows.
the surface areas of both blocks are the sum of their 6 sides each.
the special thing about seeing them as "composite figure" is that 2 sides (one for each block) are partly or even completely blocking each other off from being seen, so these parts and whole side are not part of the combined surface area.
let's start with the large block :
it is 5cm × 6cm × 20cm.
its surface area is the sum of
top and bottom 5×6
front and back 20×5
left 20×6
visible right (20 - 12)×6 = 8×6
it is (20-12)cm long, because the short block
is 12cm high.
so, we have
2 times 5×6 =2×30 = 60 cm²
2 times 20×5 = 2×100 = 200 cm²
20×6 = 120 cm²
8×6 = 48 cm²
in total : 428 cm²
now the small block :
it is 4cm × 6cm × 12cm.
its surface area is the sum of
top and bottom 4×6
front and back 4×12
no left (completely covered by the large block)
right 6×12
so, we have
2 times 4×6 = 2×24 = 48 cm²
2 times 4×12 = 2×48 = 96 cm²
6×12 = 72 cm²
in total : 216 cm²
the complete surface area of the composite figure is
428 + 216 = 644 cm²