eletric field magnitude equation: E = F/|q0| or must i use: E k|q|/r^2 ?

The magnitude of each of the charges in the figure is 9.10 10^-12 C.

The lengths of the sides of the rectangle are 3.00 cm and 6.00 cm.

Find the magnitude of the electric field at the center of the rectangle in figures a and b.

here are the figures: http://img260.imageshack.us/img260/5163/37111607os9.png

need help please.