This is an arithmetic sequence, meaning, to get the next term, you simply add some value to the current one, namely the "common difference" "d".
now, we know the 17th term is 34, let's go to the 15term then
34 + d
34 + d + d
34 + d + d + d
34 + d + d + d + d
34 + d + d + d + d + d
34 + d + d + d + d + d + d
34 + d + d + d + d + d + d + d
34 + d + d + d + d + d + d + d + d
now, notice, we got to the 15th term, which is 34 + d + d + d + d + d + d + d + d, or namely 34 + 8d, it just so happen we know is 74, therefore,
ok, now that we know what the common difference is, let's find the first term in the sequence using the 7th term of 34,
and now let's use those 2 found folks, to get the 21st term,
now, we know the 17th term is 34, let's go to the 15term then
34 + d
34 + d + d
34 + d + d + d
34 + d + d + d + d
34 + d + d + d + d + d
34 + d + d + d + d + d + d
34 + d + d + d + d + d + d + d
34 + d + d + d + d + d + d + d + d
now, notice, we got to the 15th term, which is 34 + d + d + d + d + d + d + d + d, or namely 34 + 8d, it just so happen we know is 74, therefore,
ok, now that we know what the common difference is, let's find the first term in the sequence using the 7th term of 34,
and now let's use those 2 found folks, to get the 21st term,