In the expansion of the polynomial $( x + a ) ^ { 7 }$, when the coefficient of $x ^ { 4 }$ is 280, what is the coefficient of $x ^ { 5 }$? (where $a$ is a constant) [3 points] (1) 84 (2) 91 (3) 98 (4) 105 (5) 112
In the expansion of the polynomial $( x + a ) ^ { 7 }$, when the coefficient of $x ^ { 4 }$ is 280, what is the coefficient of $x ^ { 5 }$? (where $a$ is a constant) [3 points]\\
(1) 84\\
(2) 91\\
(3) 98\\
(4) 105\\
(5) 112