Question 881938
{3, 9, 15, ...}
Arithmetic Sequence: d = 6
Note typo here
a[26] = 3 + 25d = 3 + {{{  highlight_green(150) }}} 
{{{S[n]=(n(a[1]+a[n]))/2}}}
S[26] = 26(3 + 153)/2 = 2,208