What is the 21st term of an arithmetic sequence with terms a6=50 and a11=85

A1+d (n-1)

a1+d (6-1) = 50

a1+5d=50

a1+d (11-1) = 85

a1+10d=85

a1=50-5d

(50-5d) + 10d=85

50-5d+10d=85

5d+50=85

5d=35

d=7

a1+7 (6-1) = 50

a1+35=50

a1=15

15+7 (n-1)

15+7 (21-1)

15+7 (20)

15+140=155