A firm's productivity depends on two variables, $x$ and $y$. Currently, $x=a$ and $y=b$, and the firm's productivity is 4,000 units. Productivity is increasing at a rate of 400 units per unit decrease in $x$, and is decreasing at a rate of 300 units per unit increase in $y$. What does all of this information tell you about the firm's productivity function $g(x, y)$ ?