Answer:
-24x^4 + 16x^3
Explanation:
Because exponents of a variable rely on an exponential change rather than a linear change, x^4 and x^3 are not like terms and cannot be combined. A simpler explanation is this:
2^4 + 4^8
You can't add these values, you need to simplify the exponents first. There is no way to add exponents this way without first simplifying them. So if you use variables instead of constants, simplifying the exponents is impossible and thus adding an x^4 term and x^3 term is impossible.
It really comes down to how much you can simplify it without being able to add both terms. Fortunately for you, it's really simple here. Since you're subtracting a negative, you're adding a positive! So:
-24x^4 - -16x^3 = -24x^4 + 16x^3