first we find the common difference by subtracting the first term by the second term. 75 - 69 = 6. So the common difference is 6.
now we need to find the 28th term...we need to do this because we need to use this number in the sum formula.
an = a1 + (n-1)*d n = term to find = 28 a1 = first term = 69 d = common difference = 6 now we sub a28 = 69 + (28 - 1) * 6 a28 = 69 + 27 * 6 a28 = 69 + 162 a28 = 231...this is the 28th term
now we use the sum formula sn = (n(a1 + a28) / 2 s28 = (28(69 + 231) / 2 s28 = (28(300) / 2 s28 = 8400/2 s28 = 4200 <===the sum of the first 28 terms
