Electric field is defined as force per unit charge.
So it is given by
[tex]F = q E[/tex]
now we can find electric field by
[tex]E = \frac{F}{q}[/tex]
[tex]E = \frac{4.5*10^{-4}}{6 * 10^{-4}}[/tex]
[tex]E = 0.75 N/C[/tex]
So field strength is 0.75 N/C.