[填空题]
$\text { A bird is } 4 \mathrm{~km} \text { East and } 6 \mathrm{~km} \text { North of its nest. It is flying East at a rate of } 14 \mathrm{~km} \mathrm{~h}^{-1} \text { and North at a rate of } 18 \mathrm{~km} \mathrm{~h}^{-1} \text {. Calculate the rate that its distance from the nest is changing. }$≈
