(c) If $x$ is the distance of point on the line joining the two masses from mass $m_{2}'$ where gravitational field intensity is zero, then
$\frac{Gm}{(r - x)^{2}} = \frac{Gm_{2}}{x^{2}}$ or $\frac{2}{(9 - x)^{2}} = \frac{8}{x^{2}}$
or $\frac{1}{9 - x} = \frac{2}{x}$
On solving, $x = 6m$