81a^4 - 1 = (9a^ -1)x (9a^2 +1) = (9a - 1)( 9a +1)
y^2 - x^2 -6x -9= y^2 -(x +3)^2 = ( y - x - 3)(y + x + 3)
c^3 -16c = c( c^2 - 4) = c (c - 2)(c + 2)
3a^2 -6ab +3b^2 = 3( a^2 -2ab +b^2) = 3(a -b)^2
81a^4 - 1 = (9a^ -1)x (9a^2 +1) = (9a - 1)( 9a +1)
y^2 - x^2 -6x -9= y^2 -(x +3)^2 = ( y - x - 3)(y + x + 3)
c^3 -16c = c( c^2 - 4) = c (c - 2)(c + 2)
3a^2 -6ab +3b^2 = 3( a^2 -2ab +b^2) = 3(a -b)^2