`a,\frac{11x}{2x-3}-\frac{x-18}{3-2x}`
`=\frac{11x}{2x-3}+\frac{x-18}{2x-3}`
`=\frac{11x+x-18}{2x-3}`
`=\frac{12x-18}{2x-3}`
`=\frac{6(2x-3)}{2x-3}`
`=6`
`b,\frac{-9x+18}{x^2-36}+\frac{x}{6+x}+\frac{x-3}{x-6}`
`=\frac{-9x+18+x(x-6)+(x-3)(x+6)}{(x-6)(x+6)}`
`=\frac{2x^2-12x}{(x-6)(x+6)}`
`=\frac{2x(x-6)}{(x-6)(x+6)}`
`=\frac{2x}{x+6}`