Answer
$ 50 \sqrt {7}$
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 7 and 100. 100 is a perfect square, so we know that we can simplify:
$5\sqrt {700} = 5\sqrt {100} \times \sqrt {7} = 50 \sqrt {7}$
