Nodes are points with zero amplitude while antinodes are points with maximum amplitude. The 7th harmonic has 7/4 waves.
You are attempting to create a standing wave with n = 7 in a pipe .50m in length and filled with air. The top of the pipe is open to the air and the bottom of the pipe is barely submerged in water and thus closed. Draw the diagram for this standing wave with the proper nodes and antinodes for this open-closed pipe. What is the frequency?