1 electron has a charge of 1.6 * 10^-19
2 * 10^13 electrons have a charge of x
1/2*10^13 = 1.6*10^-19 /x Cross multiply
x = 2*10^13 * 1.6*10^-19
x = 3.2 * 10^-6 coulombs.
I would say that you have to indicate that the answer is minus. You are talking about electrons.
-3.2 * 10^-6 coulombs <<<< answer