Question

What number to add to the equation to complete the square x^2+4x=0

Answer 1

4 can be the number you add to complete the square 2×2=4 and 2+2=4 (the 4 on 4x)
x^2+4x+4

Answer 2

I believe you're looking for the answer 4. If 4 is added to the equation you get x^2 +4x+4=0 which reduces to (x+2)^2