For a string fixed at both ends and plucked at the middle, the fundamental frequency is given by f0 = v/λ, where v = velocity of sound, λ = wave length. At its fundamental, note, the the length of the string is given l = λ/2, => λ0 = 2l, where λ0 = wave length fundamental note. for the first over tone, λ1 = l. For the second over tone, λ2 = 21/3 = (2 x 24)/3 = 16cm. i.e. λ2 = 16cm.