first off, let's notice that "x" and "y" are encapsulating a function in terms of time or namely "t", and that dy/dt is decreasing at a rate of 3 cm/s, since the rate is decreasing, that makes the rate negative, thus is changing at -3 cm/s.
[tex]xy=12\implies \stackrel{product~rule}{\cfrac{dx}{dt}y~~ + ~~ x\cfrac{dy}{dt}}~~ = ~~0\implies \cfrac{dx}{dt}y~~ + ~~x(-3)~~ = ~~0 \\\\\\ \cfrac{dx}{dt}y=3x\implies \left. \cfrac{dx}{dt}=\cfrac{3x}{y} \right|_{(\stackrel{x}{4},\stackrel{y}{3})}\implies \cfrac{3(4)}{3}\implies 4[/tex]
