Answer
$10-2\sqrt 7-5\sqrt 3+\sqrt {21}$
Work Step by Step
$(2-\sqrt 3)(5-\sqrt 7)$
=$2(5-\sqrt 7)-\sqrt 3(5-\sqrt 7)$
=$10-2\sqrt 7-5\sqrt 3+(\sqrt 3\sqrt 7)$
=$10-2\sqrt 7-5\sqrt 3+\sqrt {3\times7}$
=$10-2\sqrt 7-5\sqrt 3+\sqrt {21}$
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