Answer
add;
$25$
Work Step by Step
RECALL:
To complete the square for the expression $x^2 +bx$, one must add $(\frac{b}{2})^2$.
The given expression has $b=10$.
Thus, to complete the square, add $(\frac{10}{2})^2=5^2=25$ to the given expression to have:
$x^2+10x+25$
Therefore, the missing expressions in the given statement are (in order):
add;
$25$
