Remember that the slope can be written as $\frac{\text{change in } y}{\text{change in }x}$.

On this line as the $x$-value increases, does the $y$-value decrease or increase?
Decreasing makes the slope negative.
Increasing makes the slope positive.

Try answering this problem by going from the point $(−3, 3)$ to the point $(1, 0)$.