There is somemistake guys
I don?t get on what basis the man above ha written the energy equation
Even if u r wrting it u need to consider the potetial energy ??..etc which is not present above.
But the most astonishing factor is that the relation gave the correct answer.

Reliable steps to the answer
Centrifugal force is  fictiuos force.it may change from system to system.
Here qEcos(theta) acts as the centrifugal force.

That is qEcos(theta)=(mv^2) / L.

At b cos(theta)=cos 0 =1


ð qE= mv^2 / L
ð so velocity at b is v=( qEL / m ) ^1/2


tention concept introduced by the man above is difficult to digest
i didnt get what is the """"t - qe"""concept.

when a charge is put in a electric field it should have moved parallel or antiparallel to the electric field
but in this case it cant do so
this is becauz of the Tcos(theta) that prevents it from doing so

that is tcos(theta)= qE

t = qE / cos(theta)

at theta = 60 T =2qe

at b theta =0 t=qe only.

I think the question can be modified as the maximum tention possible on the string

Hope u got it

Post ur commend on the proof

???????If u don?t mind plz rate me???????