The tension in a string is $21\mathrm{.}0$ N$,$ and its linear density is $0\mathrm{.}999$ kg$/$m$.$ A wave on the string travels toward the $-$x direction; it has an amplitude of $3\mathrm{.}78$ cm and a frequency of $15\mathrm{.}2$ Hz$.$ What are the $($a$)$ speed and $($b$)$ wavelength $($in terms of m$)$ of the wave?$($c$)$ Write down a mathematical expression $($like Equation $16\mathrm{.}3$ or $16\mathrm{.}4)$ for the wave, substituting numbers for the variables A$,$ f$,$ and $.$