Rafael is analyzing data about the price of a certain stock in his finance class. he finds the linear regression line for the data to be f(x) = 0.76x + 42.39, where x is the number of days since rafael started observing the stock's price and f(x) is the price in dollars of the stock after x days of observation. if the point (15, 52.04) is in the original data set, what is the residual for when x = 15?