Recall that the standard form of the equation of a line is:
So, your slope is -500 and your y-intercept is 2,000. Draw a dot at the point (0,2000). Then place a dot at the point (1, 1500), and one at (2, 1000) all the way until it reaches the x-axis.
The largest number of brownies is simply the highest point on the graph, which happens to be 2,000. If the brownies are sold for $0, then 2,000 brownies will be sold.
The total amount of money is equal to the amount of brownies sold multiplied by the price.