Find the $x$-coordinates of all points where the graph of $f(x) = 2x^{3}+6x^{2}-90x+7$ has a horizontal tangent line.

$x$ =

Hint: What is the slope of a horizontal line?

Note: If there is more that one $x$, give the answer as a list separated by commas (e.g.: 1,2). If there are no such $x$, enter none.