The square root of a perfect square is

Identify the rational numbers.

 $3\sqrt{5}+2\sqrt{5}\ =\ ?,\ so\ the\ result\ is\ ?$  

 $4\pi\ -\ 4\pi\ =\ ?,\ which\ is\ ?$  

 $\sqrt{7}\cdot\ \sqrt{3}\ =\ ?,\ which\ is\ ?$  

 $\sqrt{7}\ \cdot\ \sqrt{7}\ =\ ?,\ which\ is\ ?$