All three-dimensional graphics have 12 sides, four long, four wide and four high. It has been made clear that it is 8 cm long, so four are 4×8=32cm and 6cm wide, so four are 4×6=24cm. If two conditions are known, subtract four lengths and four widths from the total length to get 64-32-24 = 8 cm.

8 is the length of the remaining four heights. If height is required, use 8÷4=2. So the height is 2cm, so it is adopted.