Answer:
q2 = 9.02*10^{-4}C
Step-by-step explanation:
To find the value of the other charge you use the Coulomb's law:

k: Coulomb's constant = 8.98*10^{9}Nm^2/C^2
q1: charge 1 = 55*10^{-6}C
r: distance between charges = 90cm = 0.9m
F: electric force = 550N
By doing q2 the subject of the formula and replacing you obtain:

hence, the value of the other charge q2 is 9.02*10^{-4}C