Answer:
400
Step-by-step explanation:
When completing the square, all you need to do is take half the coefficient of x, and square it.
In this case the coefficient of x is -40, and half of that is -20. -20 squared is 400, and that is what you need to add to both sides.
Note that when completing the square, you'll always be adding a positive number unless coefficient you're working with is an imaginary or complex number.
