How do you find the constant rate of change on graph?

You measure the change in the vertical direction (rise) per unit change in the horizontal direction (run).

The rate of change is constant between A and B if AB is a straight line.

Take any two points, A = (xa, ya) and B = (xb, yb)

then the average rate of change, between A and B = (yb- ya)/(xb- xa).