For this question, all you have to do is combine like terms. I am assuming that the 3x2 and 5x2 are exponents, so they are one pair and then -4x and 8x are the other. The rule is the higher exponent goes first, so here is what it looks like:
3x2 - 4x - 5x2 +8x
First combine the numbers with exponents, 3x2-5x2, you always bring over the positive or negative, which ever it may be, in this case, since it is -5x2, you bring that over.
3x2-5x2=-2x2
Now to do the number with matching letters, -4x+8x= 4x
Now just bring the problem back together:
-2x2+4x
That is your simplified answer: -2x2+4x.