The magnitude of the electric force between two obejcts with charge [tex]q_1 [/tex] and [tex]q_2[/tex] is given by Coulomb's law:
[tex]F= k_e \frac{q_1 q_2}{r^2} [/tex]
where
[tex]k_e = 8.99 \cdot 10^9 N m^2 C^{-2}[/tex] is the Coulomb's constant
and r is the distance between the two objects.
In our problem, the distance is [tex]r=12 cm=0.12 m[/tex], while the magnitudes of the two charges are
[tex]q_1 = 2.0 \mu C=2.0 \cdot 10^{-6}C[/tex]
[tex]q_2 = 3.5 \mu C = 3.5 \cdot 10^{-6} C[/tex]
(we can neglect the sign of the second charge, since we are interested only in the magnitude of the force).
So, using the formula and the data of the problem, we find
[tex]F=(8.99 \cdot 10^9 N m^2 C^{-2} ) \frac{(2.0 \cdot 10^{-6} C)(3.5 \cdot 10^{-6} C)}{(0.12 m)^2}=4.37 N [/tex]
