By using distributive property :)

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By using the F.O.I.L method:  , the question is binomial so the answer will be trinomial;

First = m²
Outer = -3
Inner = 4m
Last = -12
so the answer should be : m² - 1m - 12 . not   m² + m - 12 . .
because when you solve the Outer -3 & Inner +4 , it has different sign so the sign will be  -(negative) so that's why the result is -1m not m only . .

i think that is the answer ..... m^2-3m+4m-12=m^2+m-12