electric charge question?

Question

two small spheres each with a mass of 0.20g are suspended by light strings from a common point. They are given the same electrical charge and come to equilibrium with each string at an angle of 5 degrees with the vertical. If the string is 30 cm long what is the magnitude of the charge on each sphere?

Answer 1

Electric force Fe is balanced by a horizontal component of the weight (mg)

Fe=mgsin(5)cos(5)=kQ1Q2/R^2 since Q1=Q2
mgsin(5)=kQ^2/R^2
R=Lsin(5) where L is the length of the string
k=9 E+9 N m^2/C^2
Q= sqrt (mgsin(5) cos(5) R^2 /k)
Q= sqrt (mgsin(5) cos(5) (L sin(5))^2 /k)