Electrostatics

Three charges ' $+3 q$ ', ' $Q$ ' and ' $+q$ ' are placed in a straight line of length ' $l$ ' at points at distances $0, \frac{l}{2}$ and $l$ respectively. The value of Q in order to have the net force on +q to be zero, $\mathrm{Q}=\mathrm{xq}$. The value of $x$ is