General term of (25x3−5x21)11 is
Tr+1=Cr11(25x3)11−r(−5x21)r =Cr11(−1)r⋅211−r511−2r⋅x33−5r
The term independent of x in the expansion of (1−x2+3x3)(25x3−5x21)11 will be the coefficient of x0 in (25x3−5x21)11− coefficient of x−2 in (25x3−5x21)11+3× coefficient of x−3 in (25x3−5x21)11
=−C711(−1)7⋅245−3=53⋅24330=20033