$1.(10$pts$)$ Problem $15\mathrm{.}23($Giancoli$)$

A transverse wave on a wire is given$$by$$D$($x$,$t$)=0\mathrm{.}040$sin$(31$x$1395$t$)$where$$D$$and$$x$$are in meters and$$t$$is in seconds.

a$)$ Find an expression for a wave with the same amplitude, wavelength, and frequency but traveling in the opposite direction.

b$)$ What is the speed of either wave?