3. (24%) An elastic string of length is streched out along x-axis. The end points are fixed at x = 0 and x = x, respectively. The perpendicular displacement (») of each point follows the rule of one-dimensional wave equation. Assume that the string is released from rest with an initial displacement f(x) sintr (a) List the boundary condition(s). (4%) (b) Explain the initial condition(s) with a plot of the string. (4%) (c) With the conditions provided in (a) and (b), solve the boundary value problem of wave equation. (16%)