The slope usually indicates the behavior of a line:
In the above picture there are 3 different lines with different slopes, let's describe them:
-L1. This red line has a positive slope, it means that as "x" increases, then "y" increases as well.
-L2. This blue line has a negative slope, which means that as "x" increases, then the value of "y" decreases.
-L3. This green line is horizontal and has zero slope . This means that it doesn't matter the value of "x", "y" always has the same value.
Using this description, we can now assure that the slope in your graph is NEGATIVE