[tex]ax^{4}-x{4}+ax{3}-x{3}[/tex]

ax^4-x^4+ax^3-x^3=x^4(a-1)+x^3(a-1)=(x^4+x^3)(a-1)

(ax4 - x4) + (ax3 - x3) = x4(a-1) + x3(a-1) = (a-1)(x4 + x3)