y=mx+b the 'm' is always connected to the 'x'
your equation is just written as
y=b-mx or you can move things around
m(which is slope) = -3
The line given is in slope intercept form y =mx + b where the slope is m, and the y intercept is b. For the line y = 9 - 3x whic is the same as y = -3x + 9 the slope m = -3 and the intercept b = 9.