`a)13-\sqrt{1+5x}=8`
`<=>{(1+5x>=0),(\sqrt{1+5x}=13-8):}`
`<=>{(x>=-1/5),(1+5x=25):}`
`<=>{(x>=-1/5),(5x=24):}`
`<=>{(x>=-1/5),(x=24/5(tm)):}`
Vậy `S={24/5}`
`b)\sqrt{4x-20}+6\sqrt{\frac{x-5}{9}}-1/3\sqrt{9x-45}=6(x>=5)`
`<=>2\sqrt{x-5}+6.1/3. \sqrt{x-5}-\sqrt{x-5}=6`
`<=>2\sqrt{x-5}+2\sqrt{x-5}-\sqrt{x-5}=6`
`<=>3\sqrt{x-5}=6`
`<=>x-5=4`
`<=>x=9(tm)`
Vậy `S={9}`
