in a quadratic the number with the x^2 term is a the term with x is b and the term with no variable is c.
to complete the square take the b (which is 6 ) and divide it in half then square it.
Now add it to both sides of the equation. We can add or subtract anything we want to an equation to help us solve it, but we have to do it to both sides.
Now it should look like this:
x^2 + 6x -9 +9 = 7+9
The 9's cancel out and you can add 7 and 9 and move that over with the rest of the gang by subtracting it form both sides
x^2 +6x-16=0
factor this * remember find two terms that when multiplied together give you the c term and add to give you the b term,
(x-2)(x+8) set each side equal to 0
x-2=0 x+8=0
you get x=2 and -8