Your question:
A theater has 41 seats in the first row, 43 seats in the second row, 45 seats in the third row, and so on.
(a) Can the number of seats in each row be modeled by an arithmetic or gemetric sequence?
(b) Write the general term for a sequence a^n that gives teh number of seats in row n.
(c) How many seats are there in row 20? 

Solution:
a) The difference between the number of seats in consecutive rows is a constant,
so it is an ARITHMETIC progression. (A geometric progression is one where the
ratio of adjacent terms is a constant. E.g. 2,4,8,16...)

b) 
n = 1, row 1, number of seats = 41
n = 2, row 2, number of seats = 43 = 41 + 2*1 = 41 + d*1 (where d = 2)
n = 3, row 3, number of seats = 45 = 41 + 2*2 = 41 + d*2 (where d = 2)

So if we call the first term as a (which is 41 here), and d as the common
difference between 2 terms (which is 2 here), the nth term is
a + (n-1)*d

This is the general formula for the nth term in any arithmetic progression
with the first term as a, and common difference as d.

So number of seats in row n = 

3) How many seats in row 20? Simple - substitute n = 20 in the above formula

Row 20 has  seats

Hope you got it :)