f = fundamental frequency of violin = 441 Hz
L = original length of the violin string
v = speed of sound
Fundamental frequency is given as
f = v/(2L)
441 = v/(2L) eq-1
when the string is fingered at one-third of the way down .
L' = new length of the string to vibrate = L - (L/3) = 2L/3
f' = new fundamental frequency of the string
new fundamental frequency of the string is given as
f' = v/(2L')
f' = v/(2 (2L/3))
f' = (3/2) (v/(2L))
using eq-1
f' = (3/2) (441)
f = 661.5 Hz