Two sinusoidal waves with identical wavelengths and amplitudes travel in opposite directions along a string with a speed of 11 cm/s. If the time interval between instants when the string is flat is 0.33 s, what is the wavelength of the waves?

wavelength = (wave speed) / (frequency)
wavelength = (11 cm/s) / (0.33 s)
wavelength = 16.7 cm

Wrong answer and I know that I am missing something here but I can't figure out what. Any help?

You put time in for frequency. Hmmmm.

The time for a period is twice the .33 (think out why).

wavelength= 11cm/s * time= 11*.66 cm.