The graph below shows a scatter plot and the line of best fit relating the ages of children and the total number of times they have visited the doctor.

use the line of best fit to estimate the age of a child who has visited the doctor 40 times.

13 years old
11 years old
14 years old
12 years old

The generic equation of the line is:
 y-yo = m (x-xo)
 The slope is:
 m = (y2-y1) / (x2-x1)
 We choose ordered pairs that are closest to the line shown:
 m = (20-10) / (6-3)
 m = 10/3
 (xo, yo) = (3, 10)
 Substituting:
 y-10 = (10/3) (x-3)
 y = (10/3) x - 10 + 10
 y = (10/3) x
 For y = 10 we have:
 40 = (10/3) x
 x = (3/10) (40)
 x = 12
 
 the age of a child who has visited the doctor 40 times is:
  12 years old