It is known that the point a on the number axis represents 6, B represents - 4, and the moving point P starts from a and moves to the left at a speed of 6 unit lengths per second along the number axis at a uniform speed Given that point a on the number axis represents 6, B represents - 4, the moving point P starts from a and moves along the number axis at the speed of 6 unit lengths per second Move evenly to the left The moving point Q starts from a and moves to the left uniformly along the number axis at the speed of 1 unit length per second; the moving point R starts from B If P, Q and R move at the same time When p catches up with R, it immediately moves back to Q. when it meets Q, it stops moving. Then P moves from the beginning to the end What is the unit length of the journey when the movement stops?

It is known that the point a on the number axis represents 6, B represents - 4, and the moving point P starts from a and moves to the left at a speed of 6 unit lengths per second along the number axis at a uniform speed Given that point a on the number axis represents 6, B represents - 4, the moving point P starts from a and moves along the number axis at the speed of 6 unit lengths per second Move evenly to the left The moving point Q starts from a and moves to the left uniformly along the number axis at the speed of 1 unit length per second; the moving point R starts from B If P, Q and R move at the same time When p catches up with R, it immediately moves back to Q. when it meets Q, it stops moving. Then P moves from the beginning to the end What is the unit length of the journey when the movement stops?

Let P take x to catch up with R, and y to meet Q after returning
①、（4x）/3+6+4=6x
②、6x-x=6y+y
The equation is x = 15 / 7, y = 75 / 49
Total distance = 6 * (x + y) = 1080 / 49