Answer:
9
Explanation:
As the net force on Q is zero, so the force on Q due to q1 is balanced by the force on Q due to q2.
Let the force on Q due to q1 is F1 and force on Q due to q2 is F2.
F1 = F2
K Q q1 / (0.5 a)^2 = K Q q2 / (1.5 a)^2
q1 / 0.25 = q2 / 2.25
q2 / q1 = 2.25 / 0.25
q2 / q1 = 9