[tex]a) {4x}^{2} - {5x}^{2} - {7x}^{2} + {9x}^{2} = {x}^{2} [/tex]
[tex]b) \sqrt{18} x - \sqrt{32} x + \sqrt{50} x[/tex]
[tex] = 3 \sqrt{2} x - 4 \sqrt{2} x + 5 \sqrt{2} x[/tex]
[tex] = 4 \sqrt{2} x[/tex]
[tex]c)18 - (7x - 9) + (23x - 21)[/tex]
[tex] = 18 - 7x + 9 + 23x - 21[/tex]
[tex] = 16x + 6[/tex]
[tex] = 2(8x + 3)[/tex]
[tex]f)54 {a}^{3} {b}^{3} \div ( - 6 {a}^{2} b) = - 9a {b}^{2} [/tex]