Answer:
76,200 mm
Explanation:
1 meter = 1000 mm, by definition. Make that a conversion factor:
(1 meter)/(1000 mm), or (1000 mm)/(1 meter). Both are equal to 1, since 1 meter = 1000 mm. We can multiply anything by 1, so take the original measurement of length of 76.2 meters, and multiply it by (1000 mm)/(1 meter).
(76.2 meters)*((1000 mm)/(1 meter)) = 76,200 mm. The meters cancels, leaving just mm, the desired unit.