Why is 12 pi added at last ?

Q. Example 15.2 A wave travelling along a string is describe by,

      y (x ,t) = 0.005 sin (80.0 x – 3.0 t),

       in which the numerical constants are in SI units (0.005 m, 80.0 and m^–1, and 3.0 rad S^–1), Calculate (a) the amplitude. (b) the wavelength, and (c) the period and frequency of the wave. Also, calculate the displacement y of the wave at a distance x = 30.0 cm and time t = 20 s ?

   The displacement y at x = 30.0 cm and time t = 20 s is given by 

         y = (0.005 m ) sin ( 80.0 × 0.3 – 3.0 × 20)

             = (0.005 m) sin ( – 36 + 12 $\mathrm{\pi }$)

            = (0.00 5 m) sin (1.699)

           = (0.005 m ) sin ( 97⁰ ) $\simeq$5 mm