In the function f(x) = 5(x2 − 4x + + 15, what number belongs in the blank to complete the square

The number that would make the expression in the parentheses a perfect square is obtained by dividing the absolute value of the second term by 2 and the numerical coefficient of the first term then square the obtained number. In the given, the numerical coefficient of the first term is 1. Dividing the 4 by 1 and 2 will give 2. Squaring 2 will give four. Thus, the missing number is 4.