27x^2-81x-9x+27 In this case, just in your mind multiply 27 (coefficient of x^2) by the constant value 27(the end value of the quadratic trinomial). Then find factors that give you the value -90 when you add and 729 (27*27) when you multiply.
Factorise the values
27x( x-3)-9(x-3)
Which will give you
(27x-9)(x-3)