My answer wouldn't necessarily be in nC. It would be in Couloumbs on some order of magnitude.
I would think, oh, well I know electrons carry a charge and Coloumb is a unit of charge. I would then think how many many electrons are required to make a charge equal to a Coulomb? Then I'd look in my formula sheet and go oh here it is
6.24x10^18 electrons / Couloumb
Then if that's the number, all I have to do is group the electrons up into Couloumbs and count how many groups there are (divide).
Then I would go:
4.91x10^12 electrons / (6.24x10^18 electrons / Couloumb)
= 7.87 x 10^-9 * (electrons/electrons) * [1 / (1/Couloumbs)]
= 7.87 x 10^-9 * 1 * Couloumbs
= 7.87 x 10^-9 Couloumbs
= 7.87 nC
You know what I am doing when I divide the units right? You can treat units just like numbers. They divide and all that, even the magnitude prefixes like milli, kilo, or nano can cancel each other out across different units:
mm/ms = m/s
It works on any prefix actually as long as you take into account the difference in zeroes.
km/ms = Mm/s (M = mega)
It's just a shortcut instead of converting it to the base unit and sticking a "x10^#) at the end. The units must cancel out properly and give you right units at the end or you did something wrong...ALWAYS in every question. It's not hard...you're just getting flustered because it's a test question. Either that, or you don't understand how electrons carry a charge, Couloumbs is a unit of charge, therefore a certain amount of electrons can make a certain Couloumbs of charge.