Answer
$3 \sqrt {11}$
Work Step by Step
In order to see if a radical is in simplified form, see if any of its factors are perfect squares (meaning that their square root will be an integer). We see that 99 has factors of 11 and 9. 9 is a perfect square, so we know that we can simplify:
$\sqrt {99} = \sqrt {9} \times \sqrt {11} = 3 \sqrt {11}$
