The part of a violin string that vibrates is 0.32 m long, and each metre of it has a mass of 0.6 g…

Step 1. Wavelength and speed. $\lambda = 2L = 0.64\ \text{m}$. So $v = f\lambda = 660\ \text{Hz} \times 0.64\ \text{m} = 422.4\ \text{m/s}$.

Step 2. Tension. $v = \sqrt{T/\mu}$. We want $T$, so $T = \mu v^2 = 6 \times 10^{-4}\ \text{kg/m} \times (422.4\ \text{m/s})^2 = 107\ \text{N}$.

Step 3. Second harmonic. $2 \times 660\ \text{Hz} = 1320\ \text{Hz}$.