addition

4-2a2b+3ab2+5a2b2   +   4a2b-2a2b2+2ab2+3

In situations like these, I find it easiest just to line up like terms "vertically" and then either add or subtract....so we have.......

  4     -2a²b      +3ab²   +5a²b²

+3    +4a²b      +2ab²   -2a²b²

------------------------------------

7       +2a²b      +5ab²    +3a²b²

So we have   3a²b² +2a²b + 5ab² + 7

And that's it....