In a given linear equation, the value of the independent variable decreases at a constant rate while the value of the dependent variable increases at a constant rate. the slope of this line is

The slope is defined as the change in y/change in x ... 
The y variable is the dependent variable, the x variable is the independent variable (you plug in values of x to get your y). 
Given, the independent "x" change is increasing (or +) and the dependent "y" change is decreasing (-), 
your slope is: ( - )/ ( + ) = ( - ). 
If they were both increasing ... ( + )/ ( + ) 
or both decreasing ( - )/ ( - ) 
the slope would be positive.