DOES ANYBODY KNOW THIS??

h = 3a + 28.6
A pediatrician uses the model above to estimate the
height h of a boy, in inches, in terms of the boy's
age a, in years, between the ages of 2 and 5. Based
on the model, what is the estimated increase, in
inches, of a boy's height each year?
A) 3
B) 5.7
C) 9.5
D) 14.3