The concept that we need here to give a proper solution is mutual inductance.
The mutual inductance is given by the expression

Where,
I = current
N = Number of turns
Flux through the solenoid.
Part A) Then we have in our values that,



Replacing in the equation,


Part B) Here is required the Flux, then using the same expression we have that

We conserve the same value for the Inductance but now we have a current of 2.6, then


Therefore the flux in Solenoid 1 is 
Answer:
the extension would be less the new extension might be 3 cm
Explanation:
In a problem where a child is danger form drowning from a river who has a current of 3.1km/hr to east and the child is 0.6km fro the shore and the upstream is 2,5km from the dock. So base on the question the boat with a speed of 24.8 km/hr is 1.9 km because the child is 0.6 km off the dock so 2.5 minus 0.6