find the first 3 term s of an arithmetic sequence in which the 21 term is -54 and the fifteenth term is -170

Answer 1

given:
t(21) = a+20d = -54
t(15) = a+14d = -170
subtract them
6d = 116
d = 58/3

back into the 1st equation
a+20(58/3) = -54
a = -1322/3

so the first 3 terms are -1322/3, -1264/3, -402