Area from an antiderivative

Suppose that we have two continuous functions, $f$ and $g$, with domain $R$.

It is known that $g(x) < 0$ for all $x$ and $f'(x) = g(x)$.

Given that $f(-1) = 2$ and $f(2) = -1$, what is the area enclosed by the graph of $y = g(x)$, the $x$-axis and the lines $x = -1$ and $x = 2$?