"Rate of change" means "slope", and

(1,3), (3,5), and (4,6) are points

We must use the slope formula with all 
pairs of the three points to make sure
the rate of change is the same:

The slope formula is



Using the pair of points (1,3), and (3,5):



Using the pair of points (1,3), and (4,6):


 
Using the pair of points (3,5), and (4,6):



They are all the same, so the "slope" and 
therefore the "rate of change" is 1.

Edwin