The magnitude of the electric force exerted on a charge in an electric field is given by
[tex] F=qE [/tex]
where
q is the charge
E is the magnitude of the electric field
In this problem, we have a charge of [tex] q=6.0 \cdot 10^{-4} C [/tex], while the force exerted on it is [tex] F=4.5 \cdot 10^{-4}N [/tex], so we can rearrange the previous formula to calculate the magnitude of the electric field:
[tex] E=\frac{F}{q}=\frac{4.5 \cdot 10^{-4} N}{6.0 \cdot 10^{-4} C}=0.75 N/C [/tex]
