`{x-12}/{6x-36} + 6/{x^2-6x}`
`= {x-12}/{6(x-6)} + 6/{x(x-6)}`
`= {x^2 - 12x}/{6x(x-6)} + {36}/{6x(x-6)}`
`= {x^2 - 12x + 36}/{6x(x-6)}`
`= {(x-6)^2}/{6x(x-6)}`
`= {x-6}/{6x}`.
`1/x - 1/{x+1}`
`= {x+1}/{x.(x+1)} - x/{x(x+1)}`
`= {x+1-x}/{x.(x+1)}`
`= {1}/{x.(x+1)}`.
