Lets presume that S be the Point,

So, S $=\left( -2,4 \right)$

Then, picture under reflection within the origin S’ $=\left( 2,-4 \right)$

And picture under reflection in the y-axis S” $=\left( 2,4 \right)$

So, the distance between point of reflection $=$√$\left[ {{\left( -4-\left( -4 \right) \right)}^{2}}+{{\left( 2-2 \right)}^{2}} \right]$

= √$\left[ {{\left( 8 \right)}^{2}}+{{\left( 0 \right)}^{2}} \right]$

= √$64$

=$8units$