The revenue from the sale of x x high-end cameras is given by: r ( x ) = 1000 x − 5 x 2 r ( x ) = 1000 x - 5 x 2 a . a . what is the average change in revenue if production is changed from x = 12 x = 12 to x = 15 x = 15 ? $ $ incorrect b . b . what is the instantaneous rate of change in revenue at x = 12 x = 12 ? $ $ incorrect