Answer
(a) The frequency of Brian's string is $438.0~Hz$.
(b) Brian needs to tighten the string.
Work Step by Step
(a) We can hear beats at a frequency that is equal to the difference of the two frequencies. Since Brian's string is at a lower frequency, the frequency of Brian's string is $438.0~Hz$.
(b) To get in tune with Jennifer, the frequency of Brian's string needs to increase. If Brian tightens the string, the string will have more tension, which will increase the wave speed along the string. An increase in wave speed results in a higher frequency. Therefore, Brian needs to tighten the string.
