(x + 4)(x + 4) = ?
We know that,
(x + 4)(x + 4) = (x + 4)2
From formula, (a + b)2 = a2 + b2 + 2ab
(x + 4)2 = x2 + 42 + 2 × x × 4
= x2 + 8x + 16