options ls=80;

data a;
input y x1 x2;
cards;
42 4 2
39 4 2
48 4 3
51 4 3
49 6 2
53 6 2
61 6 3
60 6 3
;

proc reg data=a;
 model y=x1;
 model y=x2;
 model y=x1 x2/ss1 ss2;

run;