Answer
a) minimum
b) $0.68KHz$, $2.0KHz$
Work Step by Step
(a) We know that the intensity of the sound heard by Larry will be at a minimum because the sound emitted by the speaker is out of phase and the sound waves reach Larry out of phase. Thus, whatever the frequency of sound, this statement will be valid.
(b) We know that
$f_1=\frac{v}{\lambda}$
$\implies f_1=\frac{343m/s}{2(0.25m)}=0.68KHz$
and $f_2=\frac{v}{\lambda}$
$\implies f_2=\frac{343m/s}{\frac{2}{3}(0.25m)}$
$\implies f_2=2.0KHz$
