What is the rate of change for f (x) = - 0.9x+665

The rate of change is - 0.9.

Since the function f defined by f (x) = - 0.9x + 665 is in slope-intercept form

f (x) = mx + b, we identify the coefficient on x to be the slope. And the slope is the rate of change.

For every one unit increase in x, y decreases by 0.9 units.