How do you find the positions of the nodes and antinodes in harmonic physics?

I am doing a problem for harmonic physics. The problem says: A stretched string fixed at each end has a m of 40g and a length of 8.0m. The tension in the string is 49.0N. Part A says: Determine the positions of the nodes and antinodes for the 3rd harmonic. I already found part B which asks what is the vibration frequency for the harmonic. Im not sure how to begin looking for the answers to part A. If someone could tell me what equation to use so that I could do the rest on my own because I need to know what Im doing.