4 An arithmetic series has first term \(a\) and common difference \(d\).
The sum of the first 21 terms is 168 .
- Show that \(a + 10 d = 8\).
- The sum of the second term and the third term is 50 .
The \(n\)th term of the series is \(u _ { n }\).
- Find the value of \(u _ { 12 }\).
- Find the value of \(\sum _ { n = 4 } ^ { 21 } u _ { n }\).