Use cross-product antisymmetry before taking a magnitude

$\begin{aligned}(\mathbf a+2\mathbf b)\times(3\mathbf a-\mathbf b)&=3\mathbf a\times\mathbf a+6\mathbf b\times\mathbf a-\mathbf a\times\mathbf b-2\mathbf b\times\mathbf b\\&=-7\mathbf a\times\mathbf b.\end{aligned}$

Here $\mathbf a\times\mathbf a=\mathbf b\times\mathbf b=\mathbf 0$ and $\mathbf b\times\mathbf a=-\mathbf a\times\mathbf b$.

$|-7\mathbf a\times\mathbf b|=7|\mathbf a\times\mathbf b|=7(4)=28$. The intermediate vector is a negative multiple; its magnitude is positive.