$($a$)$At a certain instant, a particle$-$like object is acted on by a force$=(3\mathrm{.}40$N$)(2\mathrm{.}10$N$)+(6\mathrm{.}90$N$)$F$=(3\mathrm{.}40$N$)$i$-(2\mathrm{.}10$N$)$j$+(6\mathrm{.}90$N$)$k$$while the object's velocity is$=(1\mathrm{.}00$m$/$s$)+(3\mathrm{.}70$m$/$s$)$v$=-(1\mathrm{.}00$m$/$s$)$i$+(3\mathrm{.}70$m$/$s$)$k$.$ What is the instantaneous rate at which the force does work on the object?$($b$)$At some other time, the velocity consists of only a$$y$$component$.$ If the force is unchanged, and the instantaneous power is$-18\mathrm{.}0$W$,$what is the velocity of the object just then? $($Give your answer without a unit vector.$)$