Taking east, north and vertically upward directions along `+x`, `+y` and `+z` axes respectively.
For boat `vecV_(B)=8hati`
Relative velocity of submarine w.r.t. boat `vecV_(S//B)=12hatj-2hatk`
`:. vecV_(S)-vecV_(B)=12hatj-2hatk rArr vecV_(S)=8hati+12hatj-2hatk`
Relative velocity of helicopter w.r.t. submarine
`vecV_(H//S)=vecV_(H)-vecV_(S)`
`5hatk-4hati=vecV_(H)-(8hati+12hatj-2hatk)`
`vecV_(H)=+4hati+12hatj+3hatk`
`|vecV_(H)|=sqrt(4^(2)+12^(2)+3^(2))=13m//s`
`vecV_(H//B)=vecV_(H)-vecV_(B)=(4hati+12hatj+3hatk)-8hati= -4hati+12hatj+3hatk`
`|vecV_(H//B)|=13m//s`