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a

Let electric field is zero at a distane x from -2q charge. $\frac{K.2q}{4\pi \epsilon_0 x^2} = \frac{K 8q}{4\pi \epsilon_0 (x+L)^2}$ $\Rightarrow (x+L)^2 = 4 x^2$ $\Rightarrow (x+L) = \pm 2x$ $\Rightarrow x = L $  Distance of point from origin i.e from 8q is 2L.