If in the expansion of $(1+x{)}^m(1-x{)}^n$, the coefficients of x and $x^2$ are 3 and -6 respectively, then m is 

$\begin{array}{l}\left(1{+}^m{C}_{1}x{+}^m{C}_2{x}^2+\dots \right)\left(1{-}^n{C}_{1}x{+}^n{C}_L{x}^2-\dots \right)\\ { }^m{C}_1{-}^n{C}_1=3\Rightarrow m-n=3\\ { }^m{C}_2{+}^n{C}_2{-}^m{C_1}^n{C}_1=-6\\ m(m-1)+n(n-1)-2mn=-12\end{array}$

$\text{ or }(m-n{)}^2-(m+n)=-12$

$\text{ or }9-(2m-3)=-12\therefore m=12$