The sum of the first 20 terms of the linear sequence is 28 and the sum of the first twenty eight term is 21 find which is 0 and also the sum of the term preceding it
use your formulas.
20/2 (2a+19d) = 28
28/2 (2a+27d) = 21
solve for a and d, and then find when
a + (n-1)d = 0
and then find S_n-1 = (n-1)/2 (2a + (n-2)d)