AMC

OK. so for my lab i needed to take the partial derivative of the following equation with respect to D and y. lambda=(d*(D/y))/m lambda is the wavelength d is the distance between the slits D is the distance from the laser to the board y is the distance from the center maxima to the maxima (m=-4 to 4) and m an interger multiple of the wavelength of light ( we used -4, -3, -2, -1, 1, 2, 3, 4) I got these equations: dlambda/dD= (d/m)*y deltaD and dlambda/dy= (d/m)*D delta y You add these together and plug in variables and error to get experimental error. I understand that d, y and D are the values we used in the experiment. I also understand that delta D and delta y are the possible errors in the equipment we used to measure them. the only thing I dont understand when plugging these in is what to use for m? -Or- have i done something wrong here? Thanks I guess i forgot to mention that for every m we measured y.

a. What are the three longest wavelengths for standing waves on a 240cm long stiring that is fixed at both ends? b. if the frequency of the second longest wavelength is 50 hz, what is the frequency of the third longest wavelength? can someone walk me through how to solve this? thanks