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

Answer 1

Just use your definitions ....
"the 8 term of a linear sequence is 18" ----> a + 7d = 18
"the 12 term is 26" ----> a + 11d = 26
Now subtract them: 4d = 8
d = 4 , there! now you have your common difference
sub back into a+7d = 18 to find a

once you have a and d,
term( 20) = a + 19d = .....

Answer 2

d = (26-18)/(12-8) = 2
a_20 = a_12 + 8d = 26+16 = 42