Question

$a) 81a^{2} - 1\\b) y^{2} - x^{2} - 6x - 9$

Answer 1

$\tt a) \ \ 81a^2-1=(9a)^2-1=(9a-1)(9a+1)\\\\ b) \ \ y^2-x^2-6x-9=y^2-(x^2+6x+9)=y^2-(x+3)^2=\\{} \ \ \ \ =(y-(x+3))(y+x+3)=(y-x-3)(y+x+3)$