Given that
\(\overrightarrow{ V }_{ BC }=\overrightarrow{ V }_{ B }-\overrightarrow{ V }_{ C }\)
\(\Rightarrow\left| V _{ BC }\right|=\sqrt{ V _{ B }^2+ V _{ C }^2}\)
\(\Rightarrow 25^2=7^2+ V _{ B }^2\)
\(\Rightarrow V _{ B }^2=625-49=576\)
\(\Rightarrow V _{ B }=24\,km / hr\)