A binomial squared is written like
[tex] x^2+2ax+a^2 [/tex]
So, we have [tex]2a=6 \iff a=3 [/tex]
So, we want to complete the square [tex](x+3)^2 = x^2+6x+9[/tex]
We can add and subtract 9 to write
[tex]x^2+6x = x^2+6x+(9-9) = (x^2+6x+9)-9 = (x+3)^2-9[/tex]
