#### Answer

$4$; $16$

#### Work Step by Step

Completing the square for the $x$ group can be done by adding the square of one-half of the middle term (x-term). Thus, $(\frac{4}{2})^2=4$ will be added to both sides of the equation.
Completing the square for the $y$ group can be done by adding the square of one-half of the middle term (y-term). Thus, $(\frac{-8}{2})^2=(-4)^2=16$ will be added to both sides of the equation.
Therefore, the missing expressions in the given statements are:
$4$ and $16$, respectively.