Question

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Answer 1

$\ \ \sqrt{72} -3 \sqrt{32} \\ =\sqrt{36\cdot2} -3 \sqrt{16\cdot2} \\ =\sqrt{36}\cdot\sqrt{2} -3\cdot \sqrt{16}\cdot\sqrt{2} \\ =6\sqrt{2} -3\cdot 4\cdot\sqrt{2} \\ =6\sqrt{2} -12\sqrt{2} \\ =-6\sqrt{2}$