Can someone help me on a Relative Motion physics Question?

[azngirl92]

A river flows east to west at 3.0 m/s and is 80 m wide. A boat, capable of moving at 4.0 m/s, crosses in two different ways.
(a) Find the time to cross if the boat is pointed directly north and moves at an angle downstream.
(b) Find the time to cross if the boat is pointed at an angle upstream

 

[NascentOxygen]

What is the vector equation you have been learning about for use in all of these relative motion exercises? It involves terms such as V_a and V_{a rel b}, etc.