Use: the coal cleansing data of Exercise 15.2 to fit a model of the type E (Y) = β0 + ftjpi + β2x2 +β3x3 where the levels are x1: percent solids: 8; 12, x2: flow rate: 150: 250 gal/min, x3: pH: 5; 6 Center and scale the variables to design units. Also conduct a test for lack of fit, and comment concerning the adequacy e>f the linear regression model.