$b)_{}$ $(x^2+1)^2-4x^2_{}$
$=(x^2+1-2x)(x^2+1+2x)_{}$
$=(x^2-2x+1)(x^2+2x+1)_{}$
$=(x-1)^2.(x+1)^2_{}$
$c)_{}$ $x^2-10x-9y^2+25_{}$
$=(x^2-10x+25)-9y^2_{}$
$=(x-5)^2-(3y)^2_{}$
$=(x-5-3y)(x-5+3y)_{}$
$d)_{}$ $4x^2-36x+56_{}$
$=4.(x^2-9x+14)_{}$
$=4.(x^2-2x-7x+14)_{}$
$=4.[ x.(x-2)-7.(x-2)]_{}$
$=4.(x-2)(x-7)_{}$