Two point charges each with 0.4 C of charge are 0.3 m apart. Knowing that "k" is 9 x 10⁹ N·m²/C² (the proportionality constant for Coulomb's law), find the force between the charges.

a) 720 N
b) 2,160 N
c) 7,200 N
d) 21,600 N