The length $x$ of a rectangle is decreasing at the rate of $5 \mathrm{~cm} /$ minute and the width $y$ is increasing at the rate of $4 \mathrm{~cm} /$ minute. When $x=8 \mathrm{~cm}$ and $y=6 \mathrm{~cm}$, find the rates of change of (a) the perimeter, and (b) the area of the rectangle.