I think, that because the average is -2, and it were 3 days, it would be -6 total in 3 days, then he shot +2, that would be -4, there were for days so his final average would be -1. You would write it down like this:
$${\frac{\left({\mathtt{\,-\,}}{\mathtt{2}}{\mathtt{\,\times\,}}{\mathtt{3}}{\mathtt{\,\small\textbf+\,}}{\mathtt{2}}\right)}{{\mathtt{4}}}}$$ =
$${\frac{\left({\mathtt{\,-\,}}{\mathtt{6}}{\mathtt{\,\small\textbf+\,}}{\mathtt{2}}\right)}{{\mathtt{4}}}}$$=
$${\mathtt{\,-\,}}{\frac{{\mathtt{4}}}{{\mathtt{4}}}} = -{\mathtt{1}}$$
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