Is an ankylosaurus's armor strong enough to take a bullet?

That would depend on the type of bullet. I would say that a bullet from a handgun or a hunting rifle probably wouldn't be able to pass through the armor of an Ankylosaurus, but it would damage the armor. There are more powerful guns, though, such as certain machine guns or anti-tank weapons, which would be capable of penetrating an Ankylosaurus's armor. Of course, this is simply conjecture and one would need a lot of equipment and training to perform an experiment and determine for certain how bulletproof Ankylosaurus's armor was.