#### Answer

$7\sqrt {3rt}$

#### Work Step by Step

Step 1: $\sqrt {7r}\times\sqrt {21t}$
Step 2: $\sqrt {7r\times21t}$
Step 3: $\sqrt {147rt}$
Step 4: $\sqrt {147}\times\sqrt {rt}$
Step 5: $\sqrt {49\times3}\times\sqrt {rt}$
Step 6: $7\sqrt 3\times\sqrt {rt}$
Step 7: $7\sqrt {3rt}$