Answer
$16y^{3}$
Work Step by Step
We first combine this into one square root:
$ \sqrt {2y \times 128 y^{5}} =\sqrt {256 y^{6}}$
We now separate the number and the variable into two separate square roots:
$ \sqrt {256} \times \sqrt {y^{6}} =\sqrt {256} \times y^{3}$
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 256 is a perfect square, so we know that we can simplify:
$\sqrt {256} \times y^{3}= 16y^{3}$
