1. The problem statement, all variables and given/known data 2. At time t = 0, the displacement of an inﬁnitely long string is: y(x, t) = sin(pix/a) for −a ≤ x ≤ a The string is initially at rest. Using d’Alembert’s solution for speed c, sketch the displacement of the string at t = 0, t = a/2c, and t = a/c. 2. Relevant equations 3. The attempt at a solution So I see how to do this...my only problem is as follows: D'Alembert solutions says y = f(x-ct) + g(x+ct).. but we don't know if the wave goes left or right..so how do i know whether to sub in x-ct for x or x+ct for x? thanks!