#### Answer

$k = \frac{r-15}{2}$

#### Work Step by Step

Our goal is to isolate k. To do this we first need to subtract 15 from both sides and add 2k to both sides:
r-2k = 15
r-15-2k = 0
r-15 = 2k
We then need to divide both sides by two to get k by itself:
r-15 = 2k
$k = \frac{r-15}{2}$