Explanation
The time it takes for crest X to reach line A is three-fourths the wave’s period (the time it would take for crest X to reach the point of the next crest in the figure). Hence the wave’s period is 4 seconds. Line B is located one full wavelength plus three fourths of a second wavelength to the right of point X. Multiply that distance by the wave’s period:1(3/4)*4=7 seconds