Answer
-12 mC.
Work Step by Step
We see that the acceleration is smaller than the acceleration due to gravity alone, so the electric force acts upward. Since this is opposite the direction of the electric field, the charge on the object must be negative.
Apply Newton’s second law, with up as the positive direction.
$$F_{net}=ma$$
$$qE-mg=ma$$
$$q(-150N/C)-(1.0kg)(9.8N/kg)=(1.0kg)(-8.0m/s^2)$$
$$q=-0.012C$$
