The velocity of the rower relative to the shore is the sum of her velocity relative to the water and the velocity of the water relative to the shore:
[tex]\vec v_{R/S} = \vec v_{R/W} + \vec v_{W/S}[/tex]
Then
[tex]\vec v_{R/S} = \left(4.5\dfrac{\rm km}{\rm h},0\right) + \left(0,3.5\dfrac{\rm km}{\rm h}\right) = (4.5,3.5)\dfrac{\rm km}{\rm h}[/tex]
The magnitude of this velocity is
√(4.5² + 3.5²) m/s ≈ 5.7 m/s
