<h2>The velocity of the boat relative to an observer standing on either bank = u = 18 
</h2>
Explanation:
Let speed of the boat in still water = u
speed of the river water = v
Relative speed of the boat in the water against the river flow is given by
Upstream speed = u - v ------- (1)
⇒ u - v = 12 ------ (2)
Given that speed of the water = 6
Now velocity of the boat is given From equation (2)
⇒ u = 12 + v
Put the value of v = 6 , we get
⇒ u = 12 + 6
⇒ u = 18
therefore , the velocity of the boat relative to an observer standing on either bank = u = 18