What is the repulsive force between two pith balls that are 8 cm apart and have equal charges of -30 nC?

Common static electricity involves charges ranging from nanocoulombs to microcoulombs. How many electrons are needed to form a charge of -2.00 nC?

For (1) apply Coulomb's Law:

F = k \frac {q_1q_2}{r^2}

For your second question the charge of one electron is approximately 1.602 x 10^(-19) coulomb's. So 1 coulombs is the charge of 1/(1.69 x 10^(-19) ) electrons. Can you finish the problem from here?