Answer
$2y\sqrt {3x}$
Work Step by Step
Step 1: $\sqrt {3xy}\times\sqrt {4y}$
Step 2: $\sqrt {3xy\times4y}$
Step 3: $\sqrt {12xy^{2}}$
Step 4: $\sqrt {12}\times\sqrt x\times\sqrt {y^{2}}$
Step 5: $\sqrt {4\times3}\times\sqrt x\times y$
Step 6: $2\sqrt 3\times \sqrt x\times y$
Step 7: $2\times\sqrt 3\times \sqrt x\times y$
Step 8: $2y\sqrt {3x}$