Simplify the expression to obtain the completion of square.
[tex]\begin{gathered} x^2+16x=0 \\ x^2+2\cdot8\cdot x+(8)^2=0+(8)^2 \\ x^2+2\cdot8\cdot x+64=64 \end{gathered}[/tex]
So 64 is added on both side of equation to obtain complete square.
Answer: 64.
