x2-81 = (x-9)(x+9)
(x - 3)(x + 3)
(a + 3)(a - 3)
(9 - 5u)(9 + 5u)
(7x + 8)(7x - 9)
(x + 2)(x - 9)
9(2x - 1)(2x + 1)
The discriminant of this quadratic expression is less than zero therefore it cannot be factored.
(10x-3(10x+3)
(2y + 3)(2y - 3)
(x - 1)(2x - 9)
Since 3 squared is 9 and 9 - 9 equals 0, that's going to be tough to factor.3x2 - 9 factors to 3(x2 - 3)