Mirroring objects is just as easy as rotating objects.  Click on the object (s) that you need to mirror and select the object that will serve as the mirror line.  By just selecting the the mirror option you will only be able to mirror an object and then the original object will be deleted.
If you want to keep the original object or create multiple mirrored images, click on the object that will be mirrored, then click on the object that will be the mirror line.   Deselect the mirror line and then using the flyout, select the base point option then the mirror option then click on the copy option. Now drag the grip and click at each mirrored point.

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