To complete the square is take the second term in the expression, divided it by 2 and then squared it. This will be the number that we have to add to the original expression.
(14/2)^2=49
so, completing the expression:
x^2+14x+49
Then, the new expression can be factored into a single term squared:
x^2+14x+49= (x+7)^2
