Think about what the question is asking in a visual. The bottom of the ladder is 11 feet away from the building. That would be the run of the slope. The top of the ladder is 19 feet above the ground. That would be the rise of the slope.
Slope =

=

Since the 19 is the rise and the 11 is the run, you can simply just plug in the numbers to find the slope. This would be:

Therefore, the slope is

.
Hope this helps! :)