(II) The displacement of a transverse wave traveling on a string is represented by $D_{1}=4.2 \sin (0.84 x-47 t+2.1)$ where $D_{1}$ and $x$ are in $\mathrm{cm}$ and $t$ in $s$ . $(a)$ Find an equation that represents a wave which, when traveling in the opposite direction, will produce a standing wave when added to this one. (b) What is the equation describing the standing wave?