Two heavy spheres of masses 2M and 8M are fixed at a distance L...

Let the point of zero gravitational field be at a distance $x$ from the mass $2M$.

Equating the magnitudes of the gravitational fields due to both masses at this point:

$\dfrac{G(2M)}{x^2} = \dfrac{G(8M)}{(L-x)^2}$

$\dfrac{1}{x^2} = \dfrac{4}{(L-x)^2}$

Taking the square root of both sides:

$\dfrac{1}{x} = \dfrac{2}{L-x}$

$L - x = 2x \Rightarrow x = \dfrac{L}{3}$

The distance of the point from the mass $8M$ is $L - x = \dfrac{2L}{3}$.

The gravitational potential at this point is the sum of the potentials due to both masses:

$V = -\dfrac{G(2M)}{x} - \dfrac{G(8M)}{L-x}$

$V = -\dfrac{G(2M)}{\dfrac{L}{3}} - \dfrac{G(8M)}{\dfrac{2L}{3}}$

$V = -\dfrac{6GM}{L} - \dfrac{12GM}{L} = -\dfrac{18GM}{L}$

Answer: $-\dfrac{18GM}{L}$