(a).

Variables	Coefficients	Main effect	t-Stat	P value
Intercept	66.667	133.33	35.0717	0.0000
X 1	−2.510	−5.02	−1.4358	0.1816
X 5	5.337	10.67	3.0520	0.0122
X 6	10.927	21.85	6.2494	0.0001
X 1 X 5	−4.495	−8.99	−2.2995	0.0443
X 1 X 6	−2.898	−5.80	−1.4824	0.1690
X 5 X 6	−1.416	−2.83	−0.7245	0.4854
X 1 X 1	−10.073	−20.15	−3.0210	0.0129
X 5 X 5	−4.923	−9.85	−1.4765	0.1706
X 6 X 6	−17.636	−35.27	−5.2891	0.0004