This type of problem can be a challenge to explain without a diagram, but I'll give it my best. What you're looking at is two perpendicular vectors being combined into a single velocity. You could describe the boat's velocity as:
V = 35i - 3.5j
where i represents vectors in the parallel to the x-axis, and j represents vectors parallel to the y-axis. I made the 3.5 negative only because it is DOWNstream. All that truly matters is that you identify how you are setting it up, because remember that velocity is a vector quantity (i.e. direction matters). But since you weren't given a Cartesian correlation, you just label your set up and solve accordingly.
We get the resultant speed from Pythagorean theorem:
√[(-3.5)2 + (35)2] = √[1237.25] = 35.2 km/hr
The direction comes from the arctangent of the components:
arctan(-3.5/35) = -5.7º or in other words 5.7º downstream of straight across.