# Math Help - not sure how to do this

1. ## not sure how to do this

I am not sure how to do this can you help ?

ADD (-7m2n + 8mn2 - 12mn) + (6nm - 8n2m +9nm2)

2. Originally Posted by jim1174
I am not sure how to do this can you help ?

ADD (-7m2n + 8mn2 - 12mn) + (6nm - 8n2m +9nm2)
I understand the problem as,
$(-7m^2n+8mn^2-12mn)+(6nm-8n^2m+9nm^2)$
Open the parantheses,
$-7m^2n+8mn^2-12mn+6nm-8n^2m+9nm^2$
For example you can add $mn^2$ with $n^2m$ but not with $m^2n$.
Thus, $2nm^2-6mn$
Note, there is no $mn^2$ term because $8mn^2-8n^2m=0$