TeX source:

\begin{aligned} &=\int_{1980}^{1990} \mathrm{f}(t) \mathrm{d} t=-\{\text { area between } \mathrm{f} \text { and axis }\}  \\ &\approx-\{200 \text { ducks } / \text { year }\} \cdot\{10 \text { years }\}=-2000 \text { ducks. } \end{aligned}