Question

Net gravitational field is zero.

Answer 1

Assuming that the point lies outside both the spheres,

$\frac{GM}{r^2}=\frac{G4M}{{(12R-r)}^2}$

$\frac{12R-r}{r}=2$

$r=4R$

Hence, our assumption that the point lies outside both the spheres is correct.

Answer is $4R$.