Perform the Indicated Operation Involving Rational Expressions $\frac { n r + 9 n - 7 r - 63 } { n r + 5 n + 4 r + 20 } \cdot \frac { n ^ { 2 } - 16 } { n ^ { 3 } - 7 n ^ { 2 } }$

Short Answer

Perform the indicated operation involving rational expressions. Express final answer in simplest form. $\frac { n r + 9 n - 7 r - 63 } { n r + 5 n + 4 r + 20 } \cdot \frac { n ^ { 2 } - 16 } { n ^ { 3 } - 7 n ^ { 2 } }$