In a hockey arena, the first row has 276 seats, the second row has 288 seats and the third row has 300 seats. Each row after this continues to increase by the same number. If the arena has a total of 5 rows, how many seats are in the arena in total?
you have an AP with
a = 276
d = 12
S5 = 5/2 (2a+4d) = ____