You know that the mystery charge must be positive and between 27 and 36 cm: Let x be the distance above 27. (in m for Coulomb's law and L = .09m)
keQ(500 x 10-9 C)/x2 = 1.2 and keQ(500 x 10-9C)/(L-x)2 = 3.5
Take the ratio of these equations in order to simplify: (ratio allows us to solve for x in cm)
(9-x)2/x2 = 1.2/3.5
Now multiply out and use quadratic formula for x.
Please consider a tutor. Take care.