The following data were collected to determine the relationship between two processing variables and the hardness of a certain kind of steel:

Fit a plane by the method of least squares, and use it to estimate the average hardness of this kind of steel when the copper content is 0.14 percent and the annealing temperature is 1,100°F.

Annealing Copper content temperature (Rockwell 30-T) ( degrees F) Hardness 78.9 55.2 80.9 57.4 85.3 60.7 0.02 0.02 0.10 0.10 0.18 0.18 1,000 1,200 1,000 1,200 1,000 1.200