Given points  $(2,4,5)$ and $(3,5,-4)$ 

The ratio divided by $YZ$ plane in which it divides $\left(x_1,y_1,z_1\right) \& \left(x_2,y_2,z_2\right)$ is $=\frac{-x_1}{x_2}$

$= \frac{-2}{3}$

As the ratio is negative it will divide in $2 : 3$ externally.