Remember the rules on subtraction, we alter the sign of the subtrahend, then proceed on the rules of addition.
For example if we have:
2x^2 - (-1x^2) => 2x^2 + 1x^2
Since the signs are now the same, we add the coefficients and affix the variables and the common sign.
This is another example for the problem above:
(3x^4 - 2) - (2x^4 - x)
Change the signs of the subtrahend.
(3x^4 - 2) + (-2x^4 + x)
Put the like terms on a parentheses.
(3x^4 + (-2x^4)) + x - 2
Combine like terms.
If the signs are not the same, we get the difference and copy the sign of the larger coefficient.
3x^4 + (-2x^4) = 1x^4
So, the final answer is 1x^4 + x - 2 for the example.
You can try the same process for your problem above. :)