(6x^3+5x^2-3)+(3x^3-5x+6)=
We can only add/subtract "like" variables - variables having the same powers
So....rearranging, we have
[6x^3 + 3x^3] + 5x^2 - 5x + [6 - 3] =
3x^3 + 5x^2 - 5x + 3
And that's it.....!!!
