$($c$16$p$72)$ Four equal charges of $+4\mathrm{.}4\backslash$times $10-6$ C are placed on the corners of one face of a cube of edge length $12\mathrm{.}0$ cm$.$ A charge of $-4\mathrm{.}4\backslash$times $10-6$ C is placed at the center of the cube. What is the magnitude of the force on the charge at the center of the cube?