Answer:
C. [tex]\sqrt{3x^2+21x}[/tex]
Step-by-step explanation:
When one multiplies two radicles, one multiplies each term in the radical by the term in the other radical, then one simplifies the result and puts the radical sign over the result. If there are any perfect squares one will take the terms out. In this scenario, such is not the case.
[tex]\sqrt{3x}*\sqrt{x+7}\\\\\sqrt{3x(x+7)}\\\\\sqrt{3x^2+21x}[/tex]
