At point A 3 mm/s point B is 24 mm/s.
At point A 1 mm/s point B is 8 mm/s
So the line is increasing at a rate of 8mm/s per 1mm/s.
Knowing the rate at which it’s increasing will never change you can use
Point A times 8= Point B
300mm/s times 8= 2400mm/s
If the speed of water at point A is 300mm/s then at point B the speed of water is 2400mm/s