Guitar string i.e. string is fixed from both ends

Frequency $\propto \sqrt{ \mathrm{Tension}}$
If tension in $\mathrm{B}$ slightly decrease then frequency of $\mathrm{B}$ decrease.

If $\mathrm{B}$ is $536 \mathrm{Hz},$ the beats produced will be $536-530$ as the frequency decreases, beats with $\mathrm{A}$ also decreases.

If $\mathrm{B}$ is $524 \mathrm{Hz},$ the beats produced will be $530-524$ as the frequency decreases, beats with $\mathrm{A}$ increases.
$\therefore$ Original frequency of $\mathrm{B}$ will be $524 \mathrm{Hz}$.