Frequency of string $B=f_{B}$

Beat's frequency, $n=6 H z$

Beat's Frequency is given as$:$

$n=\left|f_{A}-f_{B}\right|$

$6=\left|324-f_{B}\right|$

$f_{B}=330 H z$ or $318 H z$

Frequency decreases with a decrease in the tension in a string. This is because frequency is directly proportional to the square root of tension. It is given as$:$

$f \propto \sqrt{T}$

Hence, the beat frequency cannot be $330 \mathrm{Hz}$

$\therefore f_{B}=318 H z$