The 8th term of a linear sequence is 18th and the 12th term is 26th .find the first term,common difference and the 20th term

Answer 1

Your question does not make sense
"The 8th term of a linear sequence is 18th"
= gobbledegoop!
In any arithmetic (linear) sequence, the terms either increase or decrease. A term can never repeat itself
if you meant:
The 8th term of a linear sequence is 18 , we can do something

Correct your question, and we'll go from there.

Answer 2

a+7d = 18
a+11d = 26
subtract them:
4d = 8
d = 2

in a+7d = 18
2+7d = 18

carry on