Final answer:
The missing length is 6 mm.
Step-by-step explanation:
To find the missing length, we need to divide the given area by the known length. So, the missing length = Area / Length. In this case, the missing length = 80.4 / 13.1 = 6.13740458015. Since the given options are in mm, we can round the answer to the nearest whole number, which is 6 mm. Therefore, the missing length is 6 mm (option a).