This is really a slope question (average rate of change)
Look at the units: millimeters per year
So millimeters/years…millimeters divided by years
The average pupil diameter changed from 4.7 to 2.3
So -2.4mm is the change but over 60 years
We know it is 60 years from 20 to 80
-2.4mm/60 years

Try the provided values by substituting in for x.  We need to be careful about using parentheses when we enter the equation into Y=
We are looking for the x value that makes the equation equal 5/6

Starts at 20 so look for y-intercept of 20
Leaves answers (2), (3) and (4)
Doubles every 10 years so must go up.
Leaves answers (3) and (4)
Which one looks like doubling every 10 years
(0, 20) is the starting point
(10,40) ten years and it doubles
(20,80) ten more years and it doubles again
(30, 160) ten more years and it doubles again
(40, 320) ten more years and it doubles again
Now we are off the graph!!

Look at the graph above to the right, where does it cross the x-axis?
Confirm these values by looking at the table below…for what x values does y = 0?
Find those values in one of the answer choices above.  TWO POINTS!!!