#### Answer

$ 42n^{2}$

#### Work Step by Step

We first combine this into one square root:
$ \sqrt {18n \times 98 n^{3}} =\sqrt {1764 n^{4}}$
We now separate the number and the variable into two separate square roots:
$ \sqrt {1764} \times \sqrt {n^{4}} =\sqrt {1764} \times n^{2}$
We see if any of the factors of a radical are perfect squares (meaning that their square root will be an integer) to see if the radical is in its most simplified form. We see that 1764 is a perfect square, so we know that we can simplify:
$\sqrt {1764} \times n^{2}= 42n^{2}$