\(\overrightarrow{ v }_{ B , l }=4\,m / s \uparrow\)
\(\overrightarrow{ v }_{ B , l }=\overrightarrow{ v }_{ B , g }-\overrightarrow{ v }_{i, g }\)
\(4\,m / s =\overrightarrow{ v }_{ B , g }-2\,m / s\)
\(\Rightarrow \overrightarrow{ v }_{ B , g }=6\,m / s \uparrow\)