a)

[tex] \frac{7}{a} + \frac{a - 7}{a + 5} = \frac{7(a + 5) + a(a - 7)}{a(a + 5)} = \\ = \frac{7a + 35 + {a}^{2} - 7a }{a {}^{2} + 5a} = \frac{ {a}^{2} + 35}{ {a}^{2} + 5a} [/tex]

б)

[tex] \frac{2 {x}^{2} }{ {x}^{2} - 4 } - \frac{2x}{x + 2} = \frac{2 {x}^{2} }{(x - 2)(x + 2)} - \frac{2x}{x + 2} = \\ = \frac{2 {x}^{2} - 2x(x - 2)}{ {x}^{2} - 4} = \frac{2 {x}^{2} - 2 {x}^{2} + 4x}{ {x}^{2} - 4 } = \frac{4x}{ {x}^{2} - 4 } [/tex]