The answer is 4.35 m/s [NE].
Since the mouse is moving relative to the cart, the cart is moving relative to the boat, and the boat is moving relative to the water, we must add up all of these velocities in order to find the speed the mouse is moving at relative to the water. But these motions are in different directions, and we must pay attention to the directions when we add these velocities.
If we sum all the velocities, we get a vector
1.7 m/s [E] + 4.0 m/s [N]
The magnitude of this vector is
v^2=(1.7)^2+(4.0)^2
which gives,
v^2=18.89
v=4.35 m/s
The direction is in between N and E.
