Tension is maintained in a string as in Figure P$13\mathrm{.}49.$ The observed wave speed is$28$m$/$s when the suspended mass is $3\mathrm{.}0$ kg$.$

$($a$)$ What is the mass per unit length of the string?

$($b$)$ What is the wave speed when the suspended mass is$1\mathrm{.}8$kg$?$