Explanation:
y = x^2 -14x + 39
take 1/2 of the x coefficient (-14) square it then add it and subtract it :
x^2 - 14x + 49 - 49 + 39 Now reduce the three Left terms
(x-7)^2 - 49 +39 simplify the R portion
y = (x-7)^2 - 10 This process is called 'completing the square'