The general form of a quadratic equation is,
Ax² + Bx + C = 0
A, B, and C are all numerical coefficients of the terms in the equation.
In order to complete the square, we divide all numerical coefficients by 2 and A. Then, square the value of B/2A. Performing this operation for this item,
n = (B/2A)²
Substituting the known values,
n = (12/2(1))² = 36
Thus, the missing number in this item is 36 and the final expression becomes,
x² + 12x + 36 = -5 + 36