Answer
$
\frac{\partial r}{\partial c} \bigg|_{b=2}
$ dollars/cow
Work Step by Step
It is given that a farmer makes $r(b, c)$ dollars of revenue from selling 6 bulls and 3 cows. The question asks us for the rate of change of the revenue when the number of cows sold is changing, and it is given that 2 bulls are sold in this case. Therefore, the partial derivative of $r$ (revenue) with respect to $c$ (number of cows) is $
\frac{\partial r}{\partial c} \bigg|_{b=2}
$
