This is called slope-intercept form. It is when a linear equation is written y=mx+b.
The m is the slope (rise over run or rate of change) and the b is the y-intercept (where this line crosses the y-axis).
So Think of rewriting this equation as:
y=-1.5x+9
Looks like y=mx+b doesn't it.
This means -1.5=m and thus the slope is -1.5.