yes in the first problem its divide so $\displaystyle \frac{10}{-2} = \frac{5}{-1} = {-5} $
in the 3rd problem
$\displaystyle u + (-21) = 18 $
this becomes $\displaystyle u -21 = 18 $ as + - becomes -
then move the -21 to the otherside of the equation
$\displaystyle u = 18 + 21$
$\displaystyle u = 39 $