`\text{a) } `

`\text{ĐKXĐ: } x \ne 2`

`\frac{x-6}{2(x-2)} + \frac{x}{x-2}`

`= \frac{x-6}{2(x-2)} + \frac{2x}{2(x-2)} `

`= \frac{x-6+2x}{2(x-2)} `

`= \frac{3x-6}{2(x-2)}`

`= \frac{3(x-2)}{2(x-2)} `

`= \frac{3}{2}`

`\text{b) } `

`\text{ĐKXĐ: } x \ne \pm y, x \ne 0`

`\frac{x-1}{x^2-y^2} \cdot \frac{x+y}{x}`

`= \frac{x-1}{(x-y)(x+y)} \cdot \frac{x+y}{x} `

`= \frac{(x-1)(x+y)}{(x-y)(x+y)x}` 

`= \frac{x-1}{x(x-y)}`

`\text{c) } `

`\text{ĐKXĐ: } x \ne \pm 2, x \ne 0`

`\frac{-5}{x^2-4} : \frac{7x^2}{x^2-2x}`

`= \frac{-5}{(x-2)(x+2)} \cdot \frac{x(x-2)}{7x^2} `

`= \frac{-5 \cdot x \cdot (x-2)}{(x-2)(x+2) \cdot 7x \cdot x} `

`= \frac{-5}{7x(x+2)} `