OpenStudy (anonymous):

For what values of x does the graph of f have a horizontal tangent? f(x) = x − 2 sin x (Use n as your integer variable. Enter your answers as a comma-separated list.)

OpenStudy (anonymous):

Horizontal tangents occur for values of $$x$$ where the slope of the tangent line is zero. This means you have to solve for all $$x$$ that make \begin{align*}f'(x)&=0\\ 1-2\cos x&=0\end{align*}