A string clamped at both ends is set into vibration by a 660 Hz...

The wavelength $\lambda$ of the transverse wave is given by the ratio of wave speed to frequency:

$\lambda = \dfrac{v}{f} = \dfrac{220}{660} = \dfrac{1}{3} \text{ m}$

For a string clamped at both ends and vibrating in $n$ loops, the length $L$ of the string is related to the wavelength by:

$L = \dfrac{n \lambda}{2}$

Given that the string vibrates in $3$ loops, $n = 3$. Substituting the values:

$L = \dfrac{3 \times \dfrac{1}{3}}{2} = \dfrac{1}{2} \text{ m}$

Converting the length to centimeters:

$L = 50 \text{ cm}$