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