An inverstor believes that the return of a certain stock depends on two factors, i.e.
   r = a + b1*f1 + b2*f2 + e.

 Year Stock return  Factor 1 Factor 2
 1     13.1           11.9      6.5
 2     9.9            15.3      1.2
 3     12.3           5.6       8.5
 4     13.1           9.7       6.7
 
 Use the least squares method to find the best constants a, b1, and b2.