x^2 - 12x = 61
Take half of the coefficient of the middle term (half of -12x), square this result, and then add, and then subtract, from -12x:
x^2 - 12x + (6)^2 - (6)^2 = 61
(x-6)^2 - 36 = 61, and so (x-6)^2 = 97
Note: These results are all equivalent; choose the one you think best.
