A study prepared for National Association of Realtors estimates that the number of housing starts in the southwest, N(t) (in units of a million), over the next 5 years is related to the mortgage rate r(t) (percent per year) by the equation + r = 36, where t is measured in years. What is the rate of change of the number of housing starts with respect to time when the mortgage rate is 11% per year and is increasing at the rate of 1.5% per year?