An air conditioning and heating repair firm conducted a study to determine if the average outside temperature, thickness of the insulation, and age

An air conditioning and heating repair firm conducted a study to determine if the average outside temperature, thickness of the insulation, and age of the heating equipment could be used to predict the electric bill for a home during the winter months in Houston, Texas. The resulting regression equation was: 

Y = 256.89 – 1.45X1 – 11.26X2 + 6.10X3, where Y = monthly cost, X1 = average temperature, X2 = insulation thickness, and X3 = age of heating equipment

Assume December has an average temperature of 45 degrees and the heater is 2 years old with insulation that is 6 inches thick.

What is the forecasted monthly electric bill?