Answer
$198+54\sqrt {7}$
Work Step by Step
$(2\sqrt {21} + 4\sqrt 3)(5\sqrt {21} -\sqrt 3)$
$(2\sqrt {21}*5\sqrt {21}+2\sqrt {21}*-\sqrt {3}+4\sqrt {3}*5\sqrt {21}+4\sqrt {3}*-\sqrt {3})$
$(10*21-2\sqrt {63}*+20\sqrt {63}-4\sqrt {9})$
$(210+18\sqrt {63}-4*3)$
$(210+18\sqrt {7*3^2}-12)$
$(198+18*3\sqrt {7})$
$(198+54\sqrt {7})$
