how to simplify (3*sqrt(6)-5)^2

 $$\\\mbox{You can expand this the long way but you should remember that}\\\\
\boxed{(a\pm b)^2=a^2\pm 2ab+b^2}\\\\

(3\sqrt{6}-5)^2\\
=(3\sqrt6)^2-[2*3\sqrt6*5]+5^2\\
=9*6-30\sqrt6+25\\
=54+25-30\sqrt6\\
=79-30\sqrt6\\$$

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