To factorise it start with putting the xs in brackets. (x )(x ) Then you must find 2 numbers which multiply to +81 and sum to -18. 81 is the product of 9Γ9 or -9Γ-9. 9+9=+18 -9+-9=-18 So they two numbers must be -9. So factorised it is (x-9)(x-9) or (x-9)^2