Explanation:
Sum of arithmetic terms = n/2 × [2a + (n - 1)×d], where 'a' is the first term, 'd' is the common difference between two numbers, and 'n' is the number of terms.
this is also the same as n×(a1 + an)/2, because
an = a1 + (n-1)×d
anyway, using that on the series above :
clearly d = 9, as every new term is the previous term plus 9.
a1 (or simply a) = 3
and we are adding up the first 33 terms, so, n = 33.
33/2 × (2×3 + 32×9) = 33/2 × (6 + 288) = 33/2 × 294 =
= 33 × 147 = 4851