In a laboratory experiment, data were gathered on the life span (Y in months) of 33 rats, units of daily protein intake (X₁), and whether or not agent X₂ (a proposed life extending agent) was added to the rats diet (X₂ = 0 if agent X₂ was not added, and X₂ = 1 if agent was added.) From the results of the experiment, the following regression model was developed.

Y=36+0.8X1X1-1.7X2X2

Also provided are SSR = 60 and SST = 180.The life expectancy of a rat that was not given any protein and that did not take agent X₂ is

I need explanation of above questions.