I need help setting up the equation

Question

For a certain species of tree, the average height of the tree after 2 years is 4 feet. After 6 years the average height is 17 feet. Assume the annual growth of the tree is linear for 2<t<29

Find the annual rate of change of the height of the tree in feet per year.

Answer 1

(2,4) and (6,17)

slope is (17-4)/(6-2) = 13/4 = 3.25 <--- rate of change per year

y = mx + b

4 = 3.25*2 + b

4 = 6.5 + b

b = -2.5

y = 3.25x - 2.5

Height = 3.25 * t - 2.5 where t is in years

Note that t=6 , Height = 3.25*6 - 2.5 = 19.5 - 2.5 = 17

3.25 feet per year