The monthly advertising revenue, A, and the monthly circulation, x, of a magazine are related approximately by the equation

where A is given in thousands of dollars and x is measured in thousands of copies sold. At what rate is the advertising revenue changing if the current circulation is x = 25 thousand copies and the circulation is growing at the rate of 2 thousand copies per month?

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A = 6√x² - 400, X x ≥ 20.