You can only combine terms with the same exponent and the same variable.
Look at it this way:
x to the fifth power terms 3x^5 - 2x^5 = x^5
x to the fourth power terms -4x^4 - -4x^4 = 0
x to the third power terms 0 - 3x^3 = -3x^3
x squared terms 2x^2 - 4x^2 = -2x^2
whole numbers 3 - -5 = 8
so you have x^5-3x^3-2x^2+8
This cannot be simplified any further.