the x squared term is "a" and the single x term is "b" and the term that doesn't have a variable is "c".
Take the "b" term divide it by 2 and then square that. This gives you the number you need to complete the square. You add this to both sides.
Here your b term is -12 so when you divide it by 2 you get -6. Squared it is 36. add this to both sides of the equal sign. so your first blank is 36 and the second blank is 61+36....97.