$
\eqalign{
  & \Delta NEC \sim \Delta CEM  \cr
  & \frac{{NE}}
{{EC}} = \frac{{CE}}
{{EM}}  \cr
  & \frac{{2r - 8}}
{4} = \frac{4}
{8} \Rightarrow 2r = 10 \Rightarrow r = 5 \cr}
$