There may be other ways.
Step 1. Let t be the time to the collision. Using a and b, write two equations on t. Dividing one equation by the other, you cat get a linear relation between a and b: they have to satisfy this relation in order for the boats to meet.
Step 2. Allowed points (a, b) are inside the circle with center (0, 0) and radius 8. The circle cuts a segment from the line obtained in step 1. Each point on the segment gives rise to some t; you need to find the point with the smallest t.
Step 3. From one of the equations from step 1 express t through a and draw a graph of t(a). Find the minimum of t when a belongs to the segment found in step 2.