Answer :
The square root of 18 = 4.24264068712. Rounding it would just be 4.2. Then 3 times 4.2 is 12.6
[tex]3\cdot \sqrt{18}=3\cdot \sqrt{9\cdot 2}=3\cdot \sqrt{9 }\cdot \sqrt{2}=3\cdot 3\cdot \sqrt{2}=9\sqrt{2}[/tex]