Two 0.10 pith balls... (HELP!)?

Question

Two 0.10g pith balls are suspended from the same point by threads 30cm long. When the balls are given equal charges, they come to rest 18cm apart. What is the magnitude of the charge on each ball? (Neglect the mass of the thread)

Answer 1

Ok, so each pith ball is pushed outward by the electric force and upward and inward by the string and pulled downward by gravity. The string must balance gravity and the electric force.
The Gravitational force is W=mg = .0001kg(9.8 m/s^2) = .00098N
Since the string length is 30 cm and the bottom of the triangle is 9 cm, the angle of the string is 17.5 degrees. Thus, the ratio (W/Felectric) = tan 17.5, or Felectric = .00098/tan 17.5 = .0031 N
Now, Felectric = k q1 q2/r^2 = 8.98E9(q1^2)/(.18 m)^2
sqrt(.0031N*(.18m)^2/(8.98*10^9 Nm^2/C*2)) = 1.06x10^-7 C
The charge on each pith ball is .000000106 Coloumbs.