Suppose the demand for Digital Video Recorders (DVRs) is given by Q=250 - .25p + 4pc, where Q is the quantity of DVRs demanded (in 1000s), p is the price of a DVR, and pc is the price of cable television. How much does Q change if the price of cable changes slightly (i.e. the partial derivative of demand with respect to pc)? please show step by step math process and explanation.