Given that P (3, 2, –4), Q (5, 4, –6) and R (9, 8, –10) are collinear.

Let point Q (5, 4, –6) divide the line segment joining points P (3, 2, –4) and R (9, 8, –10) in the ratio k:1.

Therefore, by section formula,

$(5,4,-6)=\left(\frac{k(9)+3}{k+1}, \frac{k(8)+2}{k+1}, \frac{k(-10)-4}{k+1}\right)$

$\Rightarrow \frac{9 k+3}{k+1}=5$

$\Rightarrow 9 k+3=5 k+5$

$\Rightarrow 4 k=2$

$\Rightarrow k=\frac{2}{4}=\frac{1}{2}$Thus, point Q divides PR in the ratio 1:2.