Distance between two places =$\dfrac{t\left(x^2-y^2\right)}{2x}$

Given parameters are:

Speed of boat (x) = 6 km/hr

Speed of water (y) = 4 km/hr

Time taken by the boat to go & return back = t = 3 hrs

To find the distance between the place & initial position of boat (i.e. between two places), we have

Distance between two places =$\dfrac{t\left(x^2-y^2\right)}{2x}$

=$\dfrac{3\left(6^2-4^2\right)}{2\times6}$

=$\dfrac{3\left(36-16\right)}{12}$

=$\dfrac{\left(3\times20\right)}{12}$

=5km/hr