a string is attached at one end, while the other end is vibrated by a boy. After some time vibrating the string, the boy observes a stationary wave pattern equal to the figure, and that the speed of vibration of the string is equal to twice the speed of sound. He verifies that the distance between two consecutive nodes in this pattern is 0.70m.

knowing that L is equal to the sum of all spindles, and that Vsong = 340m / s, determine the frequency of this string
A)485,7 HZ
B)680 Hz
C)856,56 HZ
D)158 Hz
E)340 Hz

someone could help me in this matter, i am trying to help my son but i was never good at physics, i will be very grateful, because i could not reach the requested values.