`a)(x-3)\sqrt{\frac{x}{9-x^2}(0<x<3)`
`=-(3-x)\sqrt{\frac{x}{9-x^2}}`
`=-\sqrt{\frac{x(3-x)^2}{(3-x)(3+x)}}`
`=-\sqrt{\frac{x(3-x)}{3+x}}`
`b)(2-a)\sqrt{\frac{2a}{a-2}}(a>2)`
`=-(a-2)\sqrt{\frac{2a}{a-2}}`
`=-\sqrt{\frac{2a(a-2)^2}{a-2}}`
`=-\sqrt{2a(a-2)}`
`=-\sqrt{2a^2-4a}`