а)с^2-16c =с(с-16) b)3a^2-16ab+3b^2=1/3(3a-8b+b*sqrt(55))(3a-8b-b*sqrt(55)) c)81a^4-1 =(9a^2-1)(9a^2+1)=(3a-1)(3a+1)(9a^2+1) d)y^2-x^2-6x-9=y^2-(x+3)^2=(y-x-3)(y+x+3)