7. Given that, for all positive integers \(n\),
$$\sum _ { r = 1 } ^ { n } ( r + a ) ( r + b ) = \frac { 1 } { 6 } n ( 2 n + 11 ) ( n - 1 )$$
where \(a\) and \(b\) are constants and \(a > b\),
- find the value of \(a\) and the value of \(b\).
- Find the value of
$$\sum _ { r = 9 } ^ { 20 } ( r + a ) ( r + b )$$