`9. `

`a)-3\sqrt{x}-7\sqrt{x}=-10\sqrt{x} `

`b)-4\sqrt{y}-10\sqrt{y}=-14\sqrt{y} `

`c)-6\sqrt{a}-9\sqrt{a}=-15\sqrt{a} `

`d)-5\sqrt{b}-12\sqrt{b}=-17\sqrt{b} `

`10. `

`a)3\sqrt{x}-4\sqrt{x}+6\sqrt{x} `

`=(3-4+6)\sqrt{x} `

`=5\sqrt{x} `

`b)-5\sqrt{x}+8\sqrt{x}-3\sqrt{x}-4\sqrt{x} `

`=(-5+8-3-4)\sqrt{x} `

`=-4\sqrt{x} `

`c)4\sqrt{x}-6\sqrt{x}-10\sqrt{x}+7\sqrt{x} `

`=(4-6-10+7)\sqrt{x} `

`=-5\sqrt{x} `

`d)-6\sqrt{x}-8\sqrt{x}-11\sqrt{x}+20\sqrt{x} `

`=(-6-8-11+20)\sqrt{x} `

`=-5\sqrt{x} `