Hi there!
If the forces are balanced, we know that:
[tex]\Sigma F = F_E - F_g = 0 \\\\F_E = F_g[/tex]
The electric field force and force due to gravity must be EQUAL.
The equation for force due to an electric field is:
[tex]F_E = qE[/tex]
q = Charge of particle (C)
E = Electric Field Strength (N/C)
The equation for force due to gravity:
[tex]F_g = W = Mg[/tex]
M = Mass (kg)
g = acceleration due to gravity (9.8 m/s²)
Set the two equal and solve for mass.
[tex]qE = Mg\\\\M = \frac{qE}{g} = \boxed{0.0021 kg}[/tex]
