#### Answer

$8 \sqrt {2}$

#### 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 128 has factors of 64 and 2. 64 is a perfect square, so we know that we can simplify:
$\sqrt {128} = \sqrt {64} \times \sqrt {2} = 8 \sqrt {2}$