[tex]a) \\ \\ 6 \sqrt{125} - 7 \sqrt{20} = \\ \\ 6 \times 5 \sqrt{5} - 7 \times 2 \sqrt{5} = \\ \\ 30 \sqrt{5} - 14 \sqrt{5} = \\ \\ 16 \sqrt{5} [/tex]
[tex]b) \\ \\ ( \frac{1}{3}) ^{3} - \frac{1}{9} = \frac{1}{ {3}^{3} } - \frac{1}{9} = \frac{1}{27} - \frac{1}{9} = - \frac{2}{27} [/tex]
