#### Answer

run=5 feet

#### Work Step by Step

We know that the definition of slope is:
$slope=\frac{rise}{run}$
In this case, we have:
$rise=15$
$run=x$
$slope=3$
Thus the slope is:
$slope=\frac{15}{x}=3$
We solve for the run, $x$:
$x=\frac{15}{3}=5$ feet