x^2 = y and rewrite your exercise
y^2 -10y +9 = y*y -y -9y +9 = y(y -1) -9(y -1) = (y -9)(y -1)
and now rewrite using this above wrote note of x^2 = y
so will get (y -9)(y -1) = (x^2 -9)(x^2 -1)
and this mean that
x^4 -10x^2 +9 = (x^2 -9(x^2 -1)
hope helped