The number of electrons in a coulomb is $6.25times10^{18}$ electrons, so the total charge of a system consisting of $3.9times10^7$ electrons is

$$
frac{3.90times10^7;{rm electrons}}{6.25times10^{18};mathrm{electrons/C}} = 6.24times10^{-12};mathrm{C}.
$$

$6.24times10^{-12}$ C.

Each carbon atom contains a number of 6 electrons, each with a negative charge of magnitude $e$. Therefore, the total negative charge in 2 moles of carbon is

$$
begin{align*}
2;{rm moles}&left(frac{6.022times10^{23};mathrm{atoms}}{mathrm{mole}}right)left(frac{6;mathrm{electrons}}{mathrm{atom}}right)left(frac{-1.60times10^{-19};mathrm{C}}{mathrm{electron}}right)\
&= -1.16times10^6;mathrm{C}.
end{align*}
$$

$-1.16times10^6$ C.