Sorry to trouble you all , but I have another question related to velocity this time.

The sides of triangle ABC in the below figure represents 3 velocities. AC (magnitude 3.6m/s) corresponds to the velocity of a boat as observed by a stationary tourist on a bridge BC ( magnitude 1.5m/s) corresponds to the velocity of a cyclist crossing the bridge once again seen by the tourist.Calculate the magnitude of the velocity vector AB , the velocity of the boat relative to the cyclist.

I can't make a head or tail of the corresponding thing they have said in this question. But even without understanding , its possible to get the answer for the first part but not so to the next part. So can someone please explain me this question? Please?