Let $f$ be the function given by $f ( x ) = \sqrt { x ^ { 4 } - 16 x ^ { 2 } }$.
(a) Find the domain of $f$.
(b) Describe the symmetry, if any, of the graph of $f$.
(c) Find $f ^ { \prime } ( x )$.
(d) Find the slope of the line normal to the graph of $f$ at $x = 5$.