To complete the square, or finding the c constant of this trinomial to make it factor evenly, use this formula to find the c constant (as according to the quadratic equation ax^2+bx+c):
(b/2)^2
When you plug in the b value, 6, you should get this:
(6/2)^2
Then, when you simplify, you get a result of 9 which would be the missing constant to fill in your trinomial. When you plug it in, you should get this as your final trinomial:
x^2+6x+9