Slope is calculated as the change in y over the change in x.
Here, the change in y is the difference between the two y-coordinates of the two points:
(-16 - 5) = -21
Find the change in x as the difference of the x coordinates with the same order as done in the y-coordinates:
-8 - 18 = -26.
Then, we have:
-21/-26 = 21/26
The slope is 21/26.