Answer
given,
frequency of the tuning fork = 260 Hz
speed of wave in the string = 640 m/s
number of loop = n = 4
Amplitude = 3.1 mm
a) wavelength of the spring
 [tex]\lambda = \dfrac{v}{f}[/tex]
 [tex]\lambda = \dfrac{640}{260}[/tex]
 [tex]\lambda =2.46\ m[/tex]
we know length of string
 [tex]L = \dfrac{n\lambda}{2}[/tex]
 [tex]L = \dfrac{4\times 2.46}{2}[/tex]
 [tex]L =4.92\ m[/tex]
b) angular frequency of standing waves
 ω = 2 π f  = 2 π x 260
 ω = 520 π rad/s
 wave number
 [tex]k =\dfrac{2\pi f}{v}[/tex]
 [tex]k =\dfrac{2\pi\times 260}{600}[/tex]
    k = 2.723 rad/m
 y (x,t) = Ym sin(kx)cos(ωt)
 y (x,t) = 3.1 sin (2.723 x) cos(520 π t)