Use y to denote the hight of the string Th en gen erally it is a function of both the time t and the coordinate z (2,x) Moreover, as a wave. it is a function as where v is the speed of the wave. So, for instance the hight of z at the time t is equal to the hight of x=0 at an earlier time t-a/v 0(-x/v)=y(t-z/)-0/v)