Perimeter rate of a triangle with changing sides

Triangle $ABC$ changes over time: $AC$ lengthens at $3$ units per second, while $AB$ and $BC$ each shorten at $2$ units per second. At time $t_1$ the sides $AC$, $AB$, $BC$ measure $12$, $14$ and $16$. At what rate (units per second) is the perimeter changing at $t_1$?

$-1$. The perimeter is the sum of the sides, so its rate is the sum of their rates, $3-2-2=-1$; the current lengths are irrelevant.