The term independent of x will come from
(1−x1+3x5)(2x2−x1)8
General term of (2x2−x1)8 is C8r(−x1)r(2x2)8−r=C8r28−r(−1)rx16−3r
From expansion
1(2x2−x1)8−x1(2x2−x1)8+3x5(2x2−x1)8
1(2x2−x1)8→ no constant term
−x1(2x2−x1)8→ for constant term
r=5⇒ −C8323(−1)5=+448
3x5(2x2−x1)8→ for constant term
r=7⇒ C38721(−1)7=48
Hence, term independent from x=448−48=400