Question

What number must you add to complete the square? x^2 + 16x = 3

Answer 1

to complete the square, take the coefficient of the x ( the non-squared x )
divide it by two
and then square the result
that is what you must add :)

Answer 2

so the coefficient of x is 16...
dividing by 2, you get 8
Square that, you get 64

And that is what you must add ! :D