Metric ?

How do you right 79,109km in smallest units?

Do you mean how to write it so the number seems smaller, or how to convert it to a smaller metric unit?

Assuming the second, you would convert 79,109 kilometers to millimeters.
1 km = 1000000mm

So just multiply 79,109 km by 1000000 to find the measurement in mm.

Barbie:

How do you right 79,109km in smallest units?

Kitty is right. Your question is very vague.

The base unit of distance in the metric system is metres.
79,109km = 79,109,000 metres

But this is not the smallest unit, nor is mm,

There are 1,000,000,000 nanomtres in a metre and there are 10 angstroms in a nanometre (Angstroms are not commonly used) These two units may be used to measure the wavelength of electroradiation or the diametre of an atom.

so 79,109km = 7.9109 x 10 ¹⁷ angstroms
That is 791,090,000,000,000,000 angstroms