Answer:
Now using
K*q3q1/(1cm)² = kq3q2/(3.5cm)²
So
q1/1²= q2/3.5²
But q2= -3nC
So q1= (1/3.5)²* 3*10-9
q1= 0.24nC
q1 will be positive because to become the net force on the q3 charge due to the remaining two charges, the force due to each charge on the third force should be equal and opposite thus q1 is positive