The library bought back a batch of books. Each shelf can hold 210 books. There are 13 bookshelves, and there are still 720 books that have not been put in. How many books did the school buy back in total?
图书室买回一批图书，每个书架可放210本书，放了13个书架，还有720本没放进去，学校一共买回多少本书？
<gadget id="calculator">210 * 13</gadget>
<output>2_730</output>

<gadget id="calculator">2_730 + 720</gadget>
<output>3_450</output>

<result>3_450</result>