Complete the square on:
x^2 - 8x
and
x^2 + 4k
Thanks
You have to add and subtract the square of half of your coefficient of $\displaystyle x$.
So $\displaystyle x^2 - 8x = x^2 - 8x + (-4)^2 - (-4)^2$
$\displaystyle = \left(x - 4\right)^2 - 16$.
$\displaystyle x^2 + 4k$ is already complete as the coefficient of $\displaystyle x$ is $\displaystyle 0$.