What do you need to add to complete the square for `a^2-6a`?

Related questions

• What is Completing the Square?
• How do you solve an equation by completing the square?
• How do you complete the square when a quadratic equation has a coefficient?
• Why is completing the square useful?
• How do you find the missing value to create a perfect square trinomial for `x^2+8x`?
• How do you solve `k^2-6k+8=0` by completing the square?
• Can every quadratic be solved by using the completing the square method?
• How do you know when to solve quadratics by factoring or completing the square?
• How do you solve `x^2+10x+9=0`?
• How do you use completing the square method to solve `4x^2+5x=-1`?