Point Q would be a nodal point. Suppose point R is a point where a wave from one source travels a distance of 2 wavelengths before meeting up with a wave from another source which travels a distance of 3 wavelengths. Point R would be a nodal point. If the path difference for points on the first nodal line is 4 cm, then the wavelength would be 6 cm. Antinodes result from constructive interference.
Nodes result from destructive interference.
The central line on the interference pattern is a nodal line. Points on nodal lines would be represented by bright spots if projected onto a screen. The path difference for points on the central antinodal line would be 0. The path difference for points on the first antinodal line would be 1 cm. If the path difference for points on the first antinodal line is 5 cm, then the path difference for points on the second antinodal line would be 7 cm.