First term: a1 = 151
common difference: d = -14 (we decrease by 14 each time, eg, 151-14 = 137)
nth term of this arithmetic sequence is...
an = a1+d(n-1)
an = 151+(-14)(n-1)
an = 151-14n+14
an = -14n+165
This will be used in the formula below
Sn = n*(a1+an)/2
Sn = n*(151+(-14n+165))/2
S26 = 26*(151+(-14*26+165))/2 ... replace every n with 26
S26 = -624
The final answer here is choice C) -624
