A price p (in dollars) and demand x (in items) for a product are related by 2x² - 5xp + 70p² = 29,200. If the price is increasing at a rate of 4 dollars per month when the price is 20 dollars, find the rate of change of the demand with respect to time. (Round your answer to four decimal places.) The rate of change of demand with respect to time is _______.