You use that equation to add up the numbers from a starting value to the last value.
"n" is the amount of terms, "a1" is the first term and "an" is the nth term, meaning it's the 64th number in the sequence.
So it would be
sum = (64(48 + 432)) / 2 which is 15360.
Is that right? Lol it's been about a year since I took advanced maths.