345nm=
i need to see how to work the problem out

3.45×10⁻⁴mm (or 0.000345mm)

Explanation:

Use a method called dimensional analysis here. It involves a chain of conversions, so we'll need some conversions to work with.

If you knew the conversion from nanometers to millimeters then you could just do it in one step. But I don't, so I won't. Anyways, you put the conversions into fraction form like so:

And then orient them in a way where multiplying the two (or more in other instances) gives you the units you want. In this cas it's millimeters so you'll have:

(345nm)•(1×10⁻⁹m/1nm)•(1mm/1×10⁻³m)

Notice how all the units reduce except for mm. From here you just multiply across and should get 345×10⁻⁶mm which simplifies to 3.45×10⁻⁴mm.