I believe you're asking about a technique in which you:
1. Move the constant to the right side of the equation: x^2 +2x = 13
2. Add the square of one half the x coefficent to both sides: here the coefficient is 2, so one half of that is 1, and 1 squared is 1: x^2 +2x + 1 =14
3. The left hand side is now a square: (x+1)^2 = 14
4. Take the square root of both sides, but realize that it may be either positive or negative: x+1 = +/- sqrt(14)
5. Get x by itself: x = -1 +/- sqrt(14)
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