The force of attraction between two charged spheres is given by Coulomb's law:
F = k * q1 * q2 / r^2
where F is the force of attraction, k is Coulomb's constant (8.99 x 10^9 Nm^2/C^2), q1 and q2 are the charges on the spheres, and r is the distance between the centers of the spheres.
Given that the force of attraction between the two spheres is 3 x 10^9 N and the distance between them is 6.0 units, we can substitute the values into Coulomb's law to find the magnitude of the charge on one of the spheres:
3 x 10^9 = k * q1 * 9 x 10^-8 / 6^2
Solving for q1, we get:
q1 = (3 x 10^9 * 6^2) / (k * 9 x 10^-8)
q1 = (3 x 10^9 * 36) / (8.99 x 10^9 * 9 x 10^-8)
q1 = 2.16 x 10^-7 C
So, the magnitude of the charge on one of the spheres is 2.16 x 10^-7 C