How to subtract square roots

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SAT Math › How to subtract square roots

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Evaluate: $\sqrt{108}-\sqrt{81}$

$6\sqrt{3}-9$

$24\sqrt{3}-81$

$24\sqrt{3}-9$

$4\sqrt{27}-\sqrt{9}$

None of the available answers

Let us factor 108 and 81

$108=2\times 54= 2\times 2\times 27= 2^2\times 3\times 9= 2^2\times 3^2\times 3$

$81=9\times 9=9^2$

$\sqrt{108}-\sqrt{81}=6\sqrt{3}-9$