Question

Determine the values of x where the tangent line is horizontal for the function:
f(x) = \frac{1}{(x^2 - 25)( x - 7)}.


The value(s) of x where the tangent line to the graph of the function is horizontal is(are) .
(If there aren't any x- values, then enter ``none'' in the answer space, and if there are more than one value, separate the values with a comma in the answer space.)