Special usage of HELP?

[Duc]

Hello everyone,
I'm having a difficult exercise to consult you about:
Rewrite sentences using suggested words.
1.''Don't worry about anything,Mrs Pitt.'' said the solicitor.''Leave it all to me.'' (the solicitor asked ....)
The correct answer is ''The solicitor asked Mrs Pitt not to worry about anything but to leave it all to him.''
I wonder why they use ''but'' here.Help me please.
_________________
Don't see your shade think you are great

[Mister Micawber]

.
No necessity, in my view:

The solicitor asked Mrs Pitt not to worry about anything and to leave it all to him.
_________________
Canadian-American native speaker
who teaches English for a living at Mister Micawber's
ESL cafe: Interview with Mr. Micawber

[Thang77]

In my opinion,the question given uses 'don't worry' and'leave' in two setences,the answer is only a sentence with both 'not to worry' and 'leave',on the other hand,'not to worry' is a negative clause but 'leave' is opposite.So i think using 'but' is correct.But is only my opinion.
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Impossible is nothing!
Self-confidence isn't enough to creat your win but without self-confidence, surely you'll lose!