I've tried working on this problem repeatedly and cannot get a reasonable answer.

I'm not quite sure what to apply; summation notation, sum of finite arithmetic series.

There are 50 rows of seats in a theater. The first row has 19 seats and each row has two more seats than the row in front of it. If 117 seats are in the last row, how many seats are in the theater?
A)1050
B)3400
C)4900
D)6800

a = 19
l = 117
n = 50

Sn = 50/2(19 + 117)

Sn = 25(136)

Sn = 3400 = B

b