So if I'm understanding you correctly, it should look like this:
k*(q1q3)/a^2=k*(q2q3)/(10+a)^2 since a < 0. I think...
So we get a = 4.14
To test this, I can calculate the force between q1 and q3 and match it with q2 and q3, since they should be equal.
So, k*(q1q3)/(4.14^2) = 23.31k
Should be correct. Any errors?