It is often easiest to get there by putting the constant on the right side of the equal sign.
.. x^2 +14x = -39
Now add the square of half the x coefficient, 7^2.
.. x^2 +14x +49 = 10
Rewrite as a square.
.. (x +7)^2 = 10
To get the solution, take the square root and subtract 7.
.. x +7 = ±√10
.. x = -7 ±√10