3 + (-5) + 8 + (-4)
= 2 +8+(-4)
=6mn i.e. the answer....
As all of them are having the same variables, we can see that they are having the same values.
But some of them are positive, while the others are negative so weea d the positive variables together first.
Now, we add the negative variables together.
now we subtract 9mn from 11mn.