An organ pipe closed at one end is 0.48 m long…

Step 1. Equal frequencies. $\dfrac{3v}{4L_c} = \dfrac{2v}{2L_o} = \dfrac{v}{L_o}$.

Step 2. Length. We want $L_o$, so $L_o = \dfrac{4L_c}{3} = \dfrac{4 \times 0.48\ \text{m}}{3} = 0.64\ \text{m}$.

Step 3. Frequency. $f = \dfrac{340\ \text{m/s}}{0.64\ \text{m}} = 531\ \text{Hz}$.