Answer:
see below
Step-by-step explanation:
given;
Planet Earth has an approximate diameter of 1.2742 x 10¹⁰ millimeters.
The sun has a diameter of 1.391 x 10⁶ kilometers.
find:
Part I:
Given that for every one kilometer there are 1,000,000 millimeters, which unit of measurement should be used to best represent the lengths of the sun and Earth's diameters? 
Part II: 
Approximate how many times greater the sun's diameter is than planet Earth's.
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part I:
the best measurement to represent the lengths of the sun and Earth's diameters is kilometers.
= planet dia. = 1.2742 x 10¹⁰ mm x  <u>          1 km         </u>    =  12,742 km   
                                                           1,000,000 mm
= sun dia. = 1.391 x 10⁶ km = 1,391 x 10³ km = 1,391,000 km
part II:
the approximate scale dia ratio of earth to sun is = sun dia. over earth's dia.
=  <u>1,391,000 km</u>
     12,742 km 
= 109.17 times
therefore,
109.17 times greater the sun's diameter is than planet Earth's.