There is more than one edexcel textbook, and there are many more exam boards than edexcel. The chances that someone will be online, on the same exam board as you, and have the same book are slim. post the question.
Guessing OP's problem is understanding what happened between the first line after differentiating and the line where the fraction comes into it. Edexcel C3 examples do tend to show random fragments of workings and I'm half asleep so I actually can't see what they've done myself, guessing they've just taken a factor of (3x-1)^-0.5 out but too tired to think.
I was having the exact same problem! What a silly thing to miss- multiplying top and bottom! I feel so stupid. Thank you so so much F1Addict, literally been racking my brain for so long!