A 384 Hz fork is held over a resonance tube…

Step 1. (a) $\lambda = 2(0.660\ \text{m} - 0.215\ \text{m}) = 0.89\ \text{m}$, so $v = 384\ \text{Hz} \times 0.89\ \text{m} = 342\ \text{m/s}$.

Step 2. (b) $e = \dfrac{0.89\ \text{m}}{4} - 0.215\ \text{m} = 0.0075\ \text{m} = 0.75\ \text{cm}$.

Step 3. (c) $384\ \text{Hz} \times 4 \times 0.215\ \text{m} = 330\ \text{m/s}$: too low, because the true quarter wavelength is $l_1 + e$.