Verified Answer :

If you put brackets around (2+1), your method of working is: 
1) 15-4(2+1)=3 
2) 15-43=3
3) 15-12=3
You don't need any more brackets, as the BIDMAS (brackets, Indices, division, multiplication, addition, subtraction) rule does the rest of the job for you. 
The answer is therefore: 15-4*(2+1)=3

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