(x - 6)(x - 6) = ?
We know that,
(x – 6)(x – 6) = (x – 6)2
From formula, (a – b)2 = a2 + b2 – 2ab
(x – 6)2 = x2 + 62 – 2 × x × 6
= x2 – 12x + 36