Question

if a3 = 20 and a6=41 for an arithmetic sequence, find a1

Answer 1

a(6)=a(1)+(6-1)d
a(3)=a(1)+(3-1)d
Subtract:
a(6)-a(3)=3d
41-20=3d
d=7

from which
a(3)=a(1)+(3-1)7
a(1)=20-14=6