The displacement of a particle on the string is given as
y = A.sin[⍵(t-x/v) + ⲫ]
where ⲫ is the phase constant.
Since a value of the phase constant will be unique for one oscillation and it will be repeated at each 2π angle. So the smallest positive value of ⲫ equivalent to 7.5π will be = 7.5π-2nπ by putting n = 3 i.e., 7.5π-6π
= 1.5π
