Answer
The frequency of the tuning fork is $~~381~Hz$
Work Step by Step
The tuning fork of unknown frequency must have a frequency difference of $3.00~Hz$ with the frequency of the standard fork.
Thus the unknown frequency of the tuning fork is either $381~Hz$ or $387~Hz$
The small piece of wax will cause the standard tuning fork to oscillate with a slightly lower frequency. Since the beat frequency decreases, the frequency difference of the tuning forks must have decreased.
Therefore, the frequency of the tuning fork is $~~381~Hz$
