A horizontal force is applied to an object making it run with a constant velocity across a surface.
1) Let's call

the speed of the southbound boat, and

the speed of the eastbound boat, which is 3 mph faster than the southbound boat. We can write the law of motion for the two boats:


2) After a time

, the two boats are

apart. Using the laws of motion written at step 1, we can write the distance the two boats covered:


The two boats travelled in perpendicular directions. Therefore, we can imagine the distance between them (45 mi) being the hypotenuse of a triangle, of which

and

are the two sides. Therefore, we can use Pythagorean theorem and write:

Solving this, we find two solutions. Discarding the negative solution, we have

, which is the speed of the southbound boat.
Answer:
After 3 seconds, the velocity is 4.5+3×1.5=9 m/s.