A polynomial function $f ( x )$ satisfies for all real numbers $x$:
$$\int _ { 1 } ^ { x } \left\{ \frac { d } { d t } f ( t ) \right\} d t = x ^ { 3 } + a x ^ { 2 } - 2$$
What is the value of $f ^ { \prime } ( a )$? (Here, $a$ is a constant.) [4 points]\\
(1) 1\\
(2) 2\\
(3) 3\\
(4) 4\\
(5) 5