Answer
$9a \sqrt {2}$
Work Step by Step
We first separate the number and the variable into two separate square roots:
$ 3\sqrt {18} \times \sqrt {a^{2}} = 3 \sqrt {18} \times a$
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 18 has factors of 2 and 9. 9 is a perfect square, so we know that we can simplify:
$3a\sqrt {18} = 3a \times \sqrt {9} \times \sqrt {2} = 9a \sqrt {2}$
