Find the Slope of the Curve at the Indicated Point $$f ( x ) = x ^ { 3 } - 15 x$$

Multiple Choice

Find the slope of the curve at the indicated point.
-Find the points where the graph of the function have horizontal tangents.
$$f ( x ) = x ^ { 3 } - 15 x$$

A) $( \sqrt { 5 } , - 30 \sqrt { 5 } )$
B) $( - 5 , - 50 ) , ( \sqrt { 5 } , 50 )$
C) $( - \sqrt { 5 } , 30 \sqrt { 5 } ) , ( 0,0 ) , ( \sqrt { 5 } , - 30 \sqrt { 5 } )$
D) $( - \sqrt { 5 } , 10 \sqrt { 5 } ) , ( \sqrt { 5 } , - 10 \sqrt { 5 } )$