Doubt from Electromagnetic Induction

electromagnetic-induction
magnetic-field
pp
easy
unsolved
#1

#2

Initial extension of spring, natural length? Recheck the question.

#3

Yes bro sorry, initial extension in spring is x

#4

So is it q_{max}= CBl x \sqrt{\frac{k}{m}}?
Just equate q/C= blv
And max velocity of AB occurs at mean position.