Need
there were originally 3 trees in an orchard. each year the owner planted the same number of trees. in the 19th year, there were 84 trees in the orchard. which function, t(n), can be used to determine the number of trees in the orchard in any year, n?

An=a1+d(n-1)
a1=first term
an=nth term
d=common differnce or how much it increases by each time
n=which one

so
originally 3 trees
a1=3

and after 19 years, 84 trees, so a19=84

a19=84=3+d(19-1)
84=3+d(18)
minus 3 both sides
81=18d
divide both sides by 18
4.5=d

an=3+4.5(n-1) or
an=4.5n-1.5
or
t(n)=3+4.5(n-1) or
t(n)=4.5n-1.5