Answer
$r=225$
Work Step by Step
The given equation is
$\Rightarrow -8=7-\sqrt r$
Add $\sqrt r+8$ to each side.
$\Rightarrow -8+\sqrt r+8=7-\sqrt r+\sqrt r+8$
Simplify.
$\Rightarrow \sqrt r=15$
Square each side of the equation.
$\Rightarrow (\sqrt{r})^2=15^2$
Simplify.
$\Rightarrow r=225$
Check $r=225$.
$\Rightarrow -8=7-\sqrt r$
$\Rightarrow -8=7-\sqrt {225}$
$\Rightarrow -8=7-\sqrt {15^2}$
$\Rightarrow -8=7-15$
$\Rightarrow -8=-8$
True.
Hence, the solution is $r=225$.
