I order to do this question, you need to put the 13 on the right side. So now you have x^2+12x+0=13. Now, take half of 12, and square it. Add this to both sides to get x^2+12x+36=49. Now factor the left side by putting the half of 12 into this blank to get, (x+_)^2=49. Final Answer is,
(x+6)^2=49.