The electrostatic force between two charged objects is given by:
[tex]F=k \frac{q_1 q_2}{r^2} [/tex]
where
k is the Coulomb's constant
q1 and q2 are the charges of the two objects
r is their separation
In this problem, the charge of each balloon is [tex]q_1=q_2=-2.06 \cdot 10^{16}C[/tex] and their separation is [tex]r=0.70 m[/tex], therefore the electrostatic force between the balloons is
[tex]F=(8.99 \cdot 10^9 Nm^2C^{-2}) \frac{(-2.06 \cdot 10^{16}C)^2}{(0.70m)^2} =7.8 \cdot 10^{42} C[/tex]
and the force is repulsive, since the two balloons have same sign charge.
