For the system to be in equilibrium net force on each charge should be zero.
Net force on charge q at the right, due to other two charge should be zero.
\(F_q=\frac{kQq}{x^ 2}+\frac{kq^ 2}{(2x)^ 2}=0\)
\(\frac{kQq}{x^2}=\frac{-kq^ 2}{4x^ 2}\)
\(\frac{Q}{q}=\frac{-1}{4}\)
