Answer:
16
Explanation:
x^2-8x+k is a quadratic expression of the form ax^2 + bx + c. Here a = 1, b = -8 and c = k. Focus on x^2-8x and complete the square as follows: Take half of the coefficient of x (that is, take half of -8) and square the result:
(-4)^2 = 16; if we now write x^2-8x+ 16, we'll have the square of (x - 4): (x -4)^2.
Thus, k = 16 turns x^2-8x+k into a perfect square.