Two polymers are ingredients in the manufacture of a synthetic fiber. The data set given in DS 13.2.2 shows the results of an experiment conducted to measure the fiber strength resulting from different concentrations of the two polymers. The polymer concentrations examined represent their standard levels, coded as 0. together with one unit above and below the standard levels. Fit the response surface model
y = β0 + β1x1 + β2x21 + β3x2 + β4x22 + β5x1x2
where the response variable y is the fiber strength, x1 is the concentration of polymer 1, and x2 is the concentration of polymer 2. At whal concentrations of the polymers would you estimate that the fiber strength is maximized?