Answer
$9.90\times10^{6}N/C$.
Work Step by Step
The weight of the mass is 1.813 N. The tension in the string is 5.18 N, so the electric force is downward, and the electric field is also pointed downward.
The net force is zero. Tension pulls upward, gravity and the electric force pull downward.
$$ F_{tension}=F_{electric}+F_{gravitational}$$
$$ 5.18N=QE+mg$$
$$E=\frac{5.18N-mg}{Q}$$
$$E=\frac{5.18N-(0.185kg)(9.80m/s^2)}{3.40\times10^{-7}C}=9.90\times10^{6}N/C$$
