Answer
The beat frequency is $3.7~Hz$
Work Step by Step
We can find the new frequency $f_n$ when the original tension $F_t$ is decreased by 2.5%.
$f_n = \frac{v}{2L}$
$f_n = \frac{1}{2L}~\sqrt{\frac{0.975F_T}{\mu}}$
$f_n = (\sqrt{0.975})~\frac{1}{2L}~\sqrt{\frac{F_T}{\mu}}$
$f_n = (\sqrt{0.975})~f$
$f_n = (\sqrt{0.975})~(294~Hz)$
$f_n = 290.3~Hz$
Since the beat frequency is the difference between the two frequencies, the beat frequency is $3.7~Hz$.