Answer
$$
0.572 \mathrm{\ m} .
$$
Work Step by Step
Since fundamental frequency for the pipe $A$ equal 300 $\mathrm{Hz}$ ,
We immediately know that second harmonic has $$f=2(300 \mathrm{Hz})=600 \mathrm{Hz}$$
Now by using, Eq. $17-39$ gives
$$
L_{A}=(2)(343 \mathrm{m} / \mathrm{s}) / 2\left(600 \mathrm{s}^{-1}\right)=0.572 \mathrm{\ m} .
$$
