Can someone help me with this question? Thanks! :)

What number must you add to complete the square x^2+12x=16

[10 points, algebra 2]according to my answer key the restrictions are -1 and 4. but why not positive 1?

Which statements are true? check all that apply. the line x = 0 is perpendicular to the line y = –3. all lines that are parallel to the y-axis are vertical lines. all lines that are perpendicular to the x-axis have a slope of 0. the equation of the line parallel to the x-axis that passes through the point (2, –6) is x = 2. the equation of the line perpendicular to the y-axis that passes through the point (–5, 1) is y = 1.