Relative velocity problem

A boat A is chasing boat B along a river by moving in the same direction as the
stream. The speed of the river stream is 6 miles per hour, the speed of boat B relative to
the river is 48 miles per hour, and boat A is 8 miles per hour slower than boat B.
Currently boat B is ahead of boat A, and a person throws a stone horizontally at boat A at a
speed 23 miles per hour relative to their boat.

What is the horizontal velocity of the stone relative
to boat A and to the river bank? [What are the positive direction of motion and indicate what the origin is]


I let the direction of boat A and B be the positive direction.
Boat A have a relative velocity of Xp=40 and boat B have a relative velocity of Xq=48.
Relative velocity of boat B to boat A Xqp=48-40=8
Am I right in letting the relative velocity of the brick Xb= -23, because its going backwards and now how do I calculate Xbp? or is that the wrong approach?

 

anyone?