What is the missing constant term in the perfect square that starts with x^2-8xx 2 - 8xx, squared, minus, 8, x?

Your notation is very confusing.

If you mean to complete the square for x2 - 8x, the answer is 16, i. e. (x - 4) 2 = x2 - 8x + 16.

If that is not what you mean, you will need to clarify your notation and re-submit your question.