Upgrade subject to access all content

Consider the function $h(x) = 7x - \cos {7x}$. Find all values of $x$ for which $h(x)$ has a relative minimum.

$x=\cfrac {3\pi}{2} +2\pi n$ where $n$ is an integer.

$x=\cfrac {3\pi}{14} +2\pi n$ where $n$ is an integer.

$x=\cfrac {(3 + 4n)\pi}{14}$ where $n$ is an integer.

$h(x)$ has no minima.