[tex](3 \sqrt{3} - 2 \sqrt{3} ) \times \sqrt{3} = \sqrt{3} \times \sqrt{3} = 3[/tex]
[tex]2 \sqrt{5} (5 \sqrt{5} - 6 \sqrt{5} ) = 2 \sqrt{5} \times ( - \sqrt{5} ) = - 10[/tex]
[tex](5 \sqrt{7} )^{2} - \sqrt{13}^{2} = 25 \times 7 - 13 = 175 - 13 = 162[/tex]
[tex](a - b)(a + b) = a^{2} - b^{2} [/tex]
