Answer
$\approx-.12$
Work Step by Step
The slope measures the ratio of the vertical change to the horizontal change between 2 points on a line.
We can write the slope as m= (vertical change)/(horizontal change)
In this example, the vertical change of the mountain is -1600 feet and the horizontal change is 2.5 miles.
Since we know that 2.5 miles is $2.5\times5280=13200$ feet
Therefore, $m=\frac{-1600}{13200}\approx-.12$